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/models/__init__.py

@@ -145,17 +145,3 @@ activity_stream_registrar.connect(WorkflowJob)
 
 # prevent API filtering on certain Django-supplied sensitive fields
 prevent_search(User._meta.get_field('password'))
-
-
-# Always, always, always defer result_stdout_text for polymorphic UnifiedJob rows
-# TODO: remove this defer in 3.3 when we implement https://github.com/ansible/ansible-tower/issues/5436
-def defer_stdout(f):
-    def _wrapped(*args, **kwargs):
-        objs = f(*args, **kwargs)
-        objs.query.deferred_loading[0].add('result_stdout_text')
-        return objs
-    return _wrapped
-
-
-for cls in UnifiedJob.__subclasses__():
-    cls.base_objects.filter = defer_stdout(cls.base_objects.filter)