Starting from Tower 3.3 and API v2, user are able to copy some existing resource objects to quickly
create new resource objects via POSTing to corresponding `/copy/` endpoint. A new `CopyAPIView` class
is introduced as the base view class for `/copy/` endpoints. It mimics the process of manually fetching
fields from the existing object to create a new object, plus the ability to automatically detect sub
structures of existing objects and make a background task-based deep copy when necessary.

## Usage
If an AWX resource is copiable, all of its object detail API views will have a related URL field
`"copy"`, which has form `/api/<version>/<resource name>/<object pk>/copy/`. GET to this endpoint
will return `can_copy`, which is a boolean indicating whether the current user can execute a copy
operation; POST to this endpoint actually copies the resource object. One field `name` is required
which will later be used as the name of the created copy. Upon success, 201 will be returned, along
with the created copy.

For some resources like credential, the copy process is not time-consuming, thus the entire copy
process will take place in the request-response cycle, and the created object copy is returned as
POST response.

For some other resources like inventory, the copy process can take longer, depending on the number
of sub-objects to copy (will explain later). Thus, although the created copy will be returned, the
copy process is not finished yet. All sub-objects (like all hosts and groups of an inventory) will
not be created until after the background copy task is finished in success.

Currently the available list of copiable resources are:

- job templates
- projects
- inventories
- workflows
- credentials
- notifications
- inventory scripts

For most of the resources above, only the object to be copied itself will be copied; For some resources
like inventories, however, sub resources belonging to the resource will also be copied to maintain the
full functionality of the copied new resource. In specific:

- When an inventory is copied, all its hosts, groups and inventory sources are copied.
- When a workflow job template is copied, all its workflow job template nodes are copied.

## How to add a copy end-point for a resource
The copy behavior of different resources largely follow the same pattern, therefore a unified way of
enabling copy capability for resources is available for developers:

Firstly, create a `/copy/` url endpoint for the target resource.

Secondly, create a view class as handler to `/copy/` endpoint. This view class should be subclassed
from `awx.api.generics.CopyAPIView`. Here is an example:
```python
class JobTemplateCopy(CopyAPIView):

    model = JobTemplate
    copy_return_serializer_class = JobTemplateSerializer
```
Note the above example declares a custom class attribute `copy_return_serializer_class`. This attribute
is used by `CopyAPIView` to render the created copy in POST response, so in most cases the value should
be the same as `serializer_class` of corresponding resource detail view, like here the value is the
`serializer_class` of `JobTemplateDetail`.

Thirdly, for the underlying model of the resource, Add 2 macros, `FIELDS_TO_PRESERVE_AT_COPY` and
`FIELDS_TO_DISCARD_AT_COPY`, as needed. Here is an example:
```python
class JobTemplate(UnifiedJobTemplate, JobOptions, SurveyJobTemplateMixin, ResourceMixin):
    '''
    A job template is a reusable job definition for applying a project (with
    playbook) to an inventory source with a given credential.
    '''
    FIELDS_TO_PRESERVE_AT_COPY = [
        'labels', 'instance_groups', 'credentials', 'survey_spec'
    ]
    FIELDS_TO_DISCARD_AT_COPY = ['vault_credential', 'credential']
```
When copying a resource object, basically all fields necessary for creating a new resource (fields
composing a valid POST body for creating new resources) are extracted from the original object and
used to create the copy.

However, sometimes we need more fields to be copied, like `credentials` of a job template, which
cannot be provided during creation. In this case we list such fields in `FIELDS_TO_PRESERVE_AT_COPY`
so that these fields won't be missed.

On the other hand, sometimes we do not want to include some fields provided in create POST body,
like `vault_credential` and `credential` fields used for creating a job template, which do not have
tangible field correspondence in `JobTemplate` model. In this case we list such fields in
`FIELDS_TO_DISCARD_AT_COPY` so that those fields won't be included.

For models that will be part of a deep copy, like hosts and workflow job template nodes, the related
POST body for creating a new object is not available. Therefore all necessary fields for creating
a new resource should also be included in `FIELDS_TO_PRESERVE_AT_COPY`.

Lastly, unit test copy behavior of the new endpoint in `/awx/main/tests/functional/test_copy.py` and
update docs (like this doc).

Fields in `FIELDS_TO_PRESERVE_AT_COPY` must be solid model fields, while fields in
`FIELDS_TO_DISCARD_AT_COPY` do not need to be. Note there are hidden fields not visible from model
definition, namely reverse relationships and fields inherited from super classes or mix-ins. A help
script `tools/scripts/list_fields.py` is available to inspect a model and list details of all its
available fields.
```
# In shell_plus
>>> from list_fields import pretty_print_model_fields
>>> pretty_print_model_fields(JobTemplate)
```

`CopyAPIView` will automatically detect sub objects of an object, and do a deep copy of all sub objects
as a background celery task. There are sometimes permission issues with sub object copy. For example,
when copying nodes of a workflow job template, there are cases where the user performing copy has no use
permission of related credential and inventory of some nodes, and it is desired those fields will be
`None`. In order to do that, developer should provide a static method `deep_copy_permission_check_func`
under corresponding specific copy view. Like
```python
class WorkflowJobTemplateCopy(WorkflowsEnforcementMixin, CopyAPIView):

    model = WorkflowJobTemplate
    copy_return_serializer_class = WorkflowJobTemplateSerializer

    # Other code

    @staticmethod
    def deep_copy_permission_check_func(user, new_objs):
        # method body

    # Other code
```
Static method `deep_copy_permission_check_func` must have and only have two arguments: `user`, the
user performing the copy; `new_objs`, a list of all sub objects of the created copy. Sub objects in
`new_objs` are initially populated disregarding any permission constraints, developer shall check
`user`'s permission against these new sub objects and react like unlink related objects or sending
warning logs. `deep_copy_permission_check_func` should not return anything.

Lastly, macro `REENCRYPTION_BLACKLIST_AT_COPY` is available as part of a model definition. It is a
list of field names which will escape re-encryption during copy. For example, `extra_data` field
of workflow job template nodes.

## Acceptance Criteria
* Credentials should be able to copy themselves. The behavior of copying credential A shall be exactly
  the same as creating a credential B with all needed fields for creation coming from credential A.
* Inventories should be able to copy themselves. The behavior of copying inventory A shall be exactly
  the same as creating an inventory B with all needed fields for creation coming from inventory A. Other
  than that, inventory B should inherit A's `instance_groups`, and have exactly the same host and group
  structures as A.
* Inventory scripts should be able to copy themselves. The behavior of copying inventory script A
  shall be exactly the same as creating an inventory script B with all needed fields for creation
  coming from inventory script A.
* Job templates should be able to copy themselves. The behavior of copying job template A
  shall be exactly the same as creating a job template B with all needed fields for creation
  coming from job template A. Other than that, job template B should inherit A's `labels`,
  `instance_groups`, `credentials` and `survey_spec`.
* Notification templates should be able to copy themselves. The behavior of copying notification
  template A shall be exactly the same as creating a notification template B with all needed fields
  for creation coming from notification template A.
* Projects should be able to copy themselves. The behavior of copying project A shall be the
  same as creating a project B with all needed fields for creation coming from project A, except for
  `local_path`, which will be populated by triggered project update. Other than that, project B
  should inherit A's `labels`, `instance_groups` and `credentials`.
* Workflow Job templates should be able to copy themselves. The behavior of copying workflow job
  template A shall be exactly the same as creating a workflow job template B with all needed fields
  for creation coming from workflow job template A. Other than that, workflow job template B should
  inherit A's `labels`, `instance_groups`, `credentials` and `survey_spec`, and have exactly the
  same workflow job template node structure as A.
* In all copy processes, `name` field of the created copy of the original object should be able to
  customize in the POST body.
* The permission for a user to make a copy for an existing resource object should be the same as the
  permission for a user to create a brand new resource object using fields from the existing object.
* The RBAC behavior of original workflow job template `/copy/` should be pertained. That is, if the
  user has no necessary permission to the related project and credential of a workflow job template
  node, the copied workflow job template node should have those fields empty.
