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

MA-1998 Install and configure django_oauth_toolkit to coexist with
existing oauth libraries.

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar