Upgrade djangorestframework to v3.1

* Upgrade edx-submissions
* Upgrade edx-ora2
* Upgrade edx-val
* Upgrade edx-proctoring
* Update all edx-platform code that depends on DRF, including:
  - auth_exchange
  - cors_csrf
  - embargo
  - enrollment
  - util
  - commerce
  - course_structure
  - discussion_api
  - mobile_api
  - notifier_api
  - teams
  - credit
  - profile_images
  - user_api
  - lib/api (OAuth2 and pagination)

cms/envs/common.py

@@ -44,7 +44,10 @@ from lms.envs.common import (
     PROFILE_IMAGE_SECRET_KEY, PROFILE_IMAGE_MIN_BYTES, PROFILE_IMAGE_MAX_BYTES,
     # The following setting is included as it is used to check whether to
     # display credit eligibility table on the CMS or not.
-    ENABLE_CREDIT_ELIGIBILITY, YOUTUBE_API_KEY
+    ENABLE_CREDIT_ELIGIBILITY, YOUTUBE_API_KEY,
+
+    # Django REST framework configuration
+    REST_FRAMEWORK,
 )
 from path import Path as path
 from warnings import simplefilter