Replace all previously text-based json fields with JSONBlob

This JSONBlob field type is a wrapper around Django's new generic
JSONField, but with the database column type forced to be text.  This
should behave close enough to our old wrapper around
django-jsonfield's JSONField and will avoid needing to do the
out-of-band database migration.

awx/main/migrations/0053_v340_workflow_inventory.py

@@ -2,10 +2,11 @@
 # Generated by Django 1.11.11 on 2018-09-27 19:50
 from __future__ import unicode_literals
 
-import awx.main.fields
 from django.db import migrations, models
 import django.db.models.deletion
 
+import awx.main.fields
+
 
 class Migration(migrations.Migration):
 
@@ -17,7 +18,7 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='workflowjob',
             name='char_prompts',
-            field=models.JSONField(blank=True, null=True, default=dict),
+            field=awx.main.fields.JSONBlob(blank=True, default=dict),
         ),
         migrations.AddField(
             model_name='workflowjob',