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:

7b9040f6 This enum was backwards
8774ff1f Use ref_name to disambiguate serializers that drf-yasg would otherwise assume are the same.
8a443971 Is this field missing because it is None?
4a1154a7 Give a safer buffer for clearing the rate limiting
64c47856 DRF 3.7.4 changed how you delegate to another view, so don't
7359ca4f Is this right? It fixes two tests
fdd66e53 Adjust the expected error message for DRF 3.7.7
9257f68f The default TIME_ZONE should be UTC