R1:c108ada7e4cb

Provide start screen and full registration flow on the new auth stuff

Summary:
Ref T1536. Code is intentionally made unreachable (see PhabricatorAuthProviderOAuthFacebook->isEnabled()).

This adds:

- A provider-driven "start" screen (this has the list of ways you can login/register).
- Registration actually works.
- Facebook OAuth works.

@chad, do you have any design ideas on the start screen? I think we poked at it before, but the big issue was that there were a limitless number of providers.