R1:db1cf41ec425

New Registration Workflow

Summary:
Currently, registration and authentication are pretty messy. Two concrete problems:

- The `PhabricatorLDAPRegistrationController` and `PhabricatorOAuthDefaultRegistrationController` controllers are giant copy/pastes of one another. This is really bad.
- We can't practically implement OpenID because we can't reissue the authentication request.

Additionally, the OAuth registration controller can be replaced wholesale by config, which is a huge API surface area and a…