R1:e1a9473eda04

Make auth provider autologin modular and implement it for all OAuth2 adapters

Summary:
Ref T10785. Around the time we launched Phacility SAAS we implemented this weird autologin hack. It works fine, so clean it up, get rid of the `instanceof` stuff, and support it for any OAuth2 provider.

(We could conceivably support OAuth1 as well, but no one has expressed an interest in it and I don't think I have any OAuth1 providers configured correctly locally so it would take a little bit to set up and test.)…