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

openedx/core/djangoapps/user_api/accounts/tests/test_views.py

@@ -551,7 +551,7 @@ class TestAccountsAPI(CacheIsolationTestCase, UserAPITestCase):
         ("level_of_education", "none", u"ȻħȺɍłɇs", u'"ȻħȺɍłɇs" is not a valid choice.'),
         ("country", "GB", "XY", u'"XY" is not a valid choice.'),
         ("year_of_birth", 2009, "not_an_int", u"A valid integer is required."),
-        ("name", "bob", "z" * 256, u"Ensure this value has at most 255 characters (it has 256)."),
+        ("name", "bob", "z" * 256, u"Ensure this field has no more than 255 characters."),
         ("name", u"ȻħȺɍłɇs", "   ", u"The name field must be at least 1 character long."),
         ("goals", "Smell the roses"),
         ("mailing_address", "Sesame Street"),

openedx/core/djangoapps/user_api/preferences/tests/test_api.py

@@ -470,7 +470,7 @@ def get_expected_validation_developer_message(preference_key, preference_value):
         preference_key=preference_key,
         preference_value=preference_value,
         error={
-            "key": [u"Ensure this value has at most 255 characters (it has 256)."]
+            "key": [u"Ensure this field has no more than 255 characters."]
         }
     )
 

openedx/core/djangoapps/user_api/serializers.py

@@ -34,6 +34,8 @@ class UserSerializer(serializers.HyperlinkedModelSerializer):
         # This list is the minimal set required by the notification service
         fields = ("id", "url", "email", "name", "username", "preferences")
         read_only_fields = ("id", "email", "username")
+        # For disambiguating within the drf-yasg swagger schema
+        ref_name = 'user_api.User'
 
 
 class UserPreferenceSerializer(serializers.HyperlinkedModelSerializer):