Auth - restore Phabricator OAuth as a provider

Summary: So meta it hurts. Fixes T887.

Test Plan: created a second instance of phabricator locally. made an account on oauth server phabricator. set up my normal dev phabricator to use this new oauth phabricator. noted the form worked. created an account via the oauth method and it worked.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T887

Differential Revision: https://secure.phabricator.com/D11287