Replace all usage of customized json fields with the Django builtin

The event_data field on event models, however, is getting an
overridden version that retains the underlying text data type for the
column, to avoid a heavy data migration on those tables.

Also, certain of the larger tables are getting these fields with the
NOT NULL constraint turned off, to avoid a long migration.

Remove the django.utils.six monkey patch we did at the beginning of
the upgrade.

awx/main/migrations/0053_v340_workflow_inventory.py

@@ -17,7 +17,7 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='workflowjob',
             name='char_prompts',
-            field=awx.main.fields.JSONField(blank=True, default=dict),
+            field=models.JSONField(blank=True, null=True, default=dict),
         ),
         migrations.AddField(
             model_name='workflowjob',