Modify forum roles query for Aurora performance.

This is a fix for the performance issues in EDUCATOR-3374

AWS's Aurora backend for MySQL selects the wrong index on the
django_comment_client_role_users table, leading to performance issues.
This commit replaces that join with individual requests for permissions
for each role (of which there may be several for any given user).
It's dumber SQL, but Aurora will do the right thing.

lms/djangoapps/courseware/tests/test_discussion_xblock.py

@@ -406,7 +406,7 @@ class TestXBlockQueryLoad(SharedModuleStoreTestCase):
         # * django_comment_client_role
         # * django_comment_client_permission
         # * lms_xblock_xblockasidesconfig
-        num_queries = 3
+        num_queries = 2
         for discussion in discussions:
             discussion_xblock = get_module_for_descriptor_internal(
                 user=user,