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/0020_v330_instancegroup_policies.py

@@ -1,8 +1,10 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
+from decimal import Decimal
 
 from django.db import migrations, models
-from decimal import Decimal
+
+import awx.main.fields
 
 
 class Migration(migrations.Migration):
@@ -15,7 +17,7 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='instancegroup',
             name='policy_instance_list',
-            field=models.JSONField(
+            field=awx.main.fields.JSONBlob(
                 default=list, help_text='List of exact-match Instances that will always be automatically assigned to this group', blank=True
             ),
         ),