defer UnifiedJob.result_stdout_text for improved performance

result_stdout_text can be _very_ large - some customers have 5MB+ per
job; querying for this in list contexts results in _very_ large datasets
being read from the database which is very slow.  It's very uncommon to
actually need this column outside of the context of job details, so
defer it.

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

awx/main/access.py

@@ -105,7 +105,14 @@ def get_user_capabilities(user, instance, **kwargs):
     convenient for the user interface to consume and hide or show various
     actions in the interface.
     '''
-    access_class = access_registry[instance.__class__]
+    cls = instance.__class__
+    # When `.defer()` is used w/ the Django ORM, the result is a subclass of
+    # the original that represents e.g.,
+    # awx.main.models.ad_hoc_commands.AdHocCommand_Deferred_result_stdout_text
+    # We want to do the access registry lookup keyed on the base class name.
+    if getattr(cls, '_deferred', False):
+        cls = instance.__class__.__bases__[0]
+    access_class = access_registry[cls]
     return access_class(user).get_user_capabilities(instance, **kwargs)
 
 
@@ -2071,6 +2078,7 @@ class UnifiedJobAccess(BaseAccess):
         #    'job_template__project',
         #    'job_template__credential',
         #)
+        qs = qs.defer('result_stdout_text')
         return qs.all()