move legacy UnifiedJob stdout data to a separate unmanaged model

This data often (in the case of inventory updates) represents large data
blobs (5+MB per job run).  Storing it on the polymorphic base class
table, `main_unifiedjob`, causes it to be automatically fetched on every
query (and every polymorphic join) against that table, which can result
in _very_ poor performance for awx across the board.  Django offers
`defer()`, but it's quite complicated to sprinkle this everywhere (and
easy to get wrong/introduce side effects related to our RBAC and usage
of polymorphism).

This change moves the field definition to a separate unmanaged model
(which references the same underlying `main_unifiedjob` table) and adds
a proxy for fetching the data as needed

see https://github.com/ansible/awx/issues/200

awx/main/migrations/0013_move_deprecated_stdout.py

@@ -0,0 +1,44 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.7 on 2017-12-12 18:56
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0012_non_blank_workflow'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='unifiedjob',
+            name='result_stdout_text',
+            field=models.TextField(editable=False, null=True),
+        ),
+        # Using SeparateDatabaseAndState here allows us to update the migration
+        # state so that Django thinks the UnifiedJob.result_stdout_text field
+        # is gone _without_ actually deleting the underlying column/data
+        migrations.SeparateDatabaseAndState(state_operations=[
+            migrations.RemoveField(
+                model_name='unifiedjob',
+                name='result_stdout_text',
+            ),
+        ]),
+        # On other side of the equation, this migration introduces a new model
+        # which is *unmanaged* (meaning, a new table is not created for it);
+        # instead, this sort of "virtual" model is used to maintain an ORM
+        # reference to the actual `main_unifiedjob.result_stdout_text` column
+        migrations.CreateModel(
+            name='UnifiedJobDeprecatedStdout',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('result_stdout_text', models.TextField(editable=False, null=True))
+            ],
+            options={
+                'db_table': 'main_unifiedjob',
+                'managed': False,
+            },
+        ),
+    ]