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/conf/models.py

@@ -8,6 +8,7 @@ import json
 from django.db import models
 
 # AWX
+from awx.main.fields import JSONBlob
 from awx.main.models.base import CreatedModifiedModel, prevent_search
 from awx.main.utils import encrypt_field
 from awx.conf import settings_registry
@@ -18,7 +19,7 @@ __all__ = ['Setting']
 class Setting(CreatedModifiedModel):
 
     key = models.CharField(max_length=255)
-    value = models.JSONField(null=True)
+    value = JSONBlob(null=True)
     user = prevent_search(models.ForeignKey('auth.User', related_name='settings', default=None, null=True, editable=False, on_delete=models.CASCADE))
 
     def __str__(self):