PSA was monolothic, now split, with new features, like
a DB-backed partial pipeline. FB OAuth2 version also upped.
Partial pipelines don't get cleared except when necessary.
They persist for special cases like change of browser while
still mid-pipeline (i.e. email validation step).
Refactor, cleanup, and update of a lot of small things as well.
PLEASE NOTE the new `social_auth_partial` table.
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
