Migrate to latest, split python-social-auth.

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.