Auth - allow for "auto login" providers

Summary: Ref T7153. I am not sure if this is 100% correct because sometimes you have to POST vs GET and I don't know if the redirect response will / can do the right thing? I think options to fix this would be to 1) restrict this functionality to JUST the Phabricator OAuth provider type or 2) something really fancy with an HTTP(S) future. The other rub right now is when you logout you get half auto-logged in again... Thoughts on that?

Test Plan: setup my local instance to JUST have phabricator oauth available to login. was presented with the dialog automagically...!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7153

Differential Revision: https://secure.phabricator.com/D11701