trick django-polymorphic into allowing defer() on polymorphic objects

django-polymorphic itself generates queries for polymorphic object
lookups, and these queries for UnifiedJob are *not* properly defering the
`result_stdout_text` column, resulting in more very slow queries.  This
solution is _very_ hacky, and very specific to this specific
version of Django and django-polymorphic, but it works until we can
solve this problem the proper way in 3.3 (by removing large stdout blobs
from the database).

see: https://github.com/ansible/ansible-tower/issues/7568

awx/main/models/__init__.py

@@ -145,3 +145,15 @@ 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
+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)