Fix problems that drf-yasg uncovered.

These were originally fixed individually, but had to be reverted, and
are now combined in one commit.  The originals were:

7b9040f6b0 This enum was backwards
8774ff1f9b Use ref_name to disambiguate serializers that drf-yasg would otherwise assume are the same.
8a44397139 Is this field missing because it is None?
4a1154a7ca Give a safer buffer for clearing the rate limiting
64c47856dd DRF 3.7.4 changed how you delegate to another view, so don't
7359ca4fb2 Is this right? It fixes two tests
fdd66e5390 Adjust the expected error message for DRF 3.7.7
9257f68fd8 The default TIME_ZONE should be UTC

lms/djangoapps/discussion/rest_api/api.py

@@ -61,6 +61,7 @@ from openedx.core.djangoapps.django_comment_common.signals import (
     thread_voted
 )
 from openedx.core.djangoapps.django_comment_common.utils import get_course_discussion_settings
+from openedx.core.djangoapps.user_api.accounts.api import get_account_settings
 from openedx.core.djangoapps.user_api.accounts.views import AccountViewSet
 from openedx.core.lib.exceptions import CourseNotFoundError, DiscussionNotFoundError, PageNotFoundError
 
@@ -365,10 +366,11 @@ def _get_user_profile_dict(request, usernames):
 
         A dict with username as key and user profile details as value.
     """
-    request.GET = request.GET.copy()  # Make a mutable copy of the GET parameters.
-    request.GET['username'] = usernames
-    user_profile_details = AccountViewSet.as_view({'get': 'list'})(request).data
-
+    if usernames:
+        username_list = usernames.split(",")
+    else:
+        username_list = []
+    user_profile_details = get_account_settings(request, username_list)
     return {user['username']: user for user in user_profile_details}