Available backends:
* django-oauth-toolkit (DOT)
* django-oauth2-provider (DOP)
* Use provided client ID to select backend for
* AccessToken requests
* third party auth-token exchange
* Create adapters to isolate library-dependent functionality
* Handle django-oauth-toolkit tokens in edX DRF authenticator class
MA-1998
MA-2000
Format responses as a JSON dict containing an error_code and a
developer_message. user_message is not necessary, as these are API
level errors, not seen by end users.
MA-1900
