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. Today, we have:

  - Password
  - LDAP
  - Facebook
  - GitHub
  - Phabricator
  - Disqus
  - Google

We plan to add:

  - Asana
  - An arbitrary number of additional instances of Phabricator

Users want to add:

  - OpenID
  - Custom providers

And I'd like to have these at some point:

  - Stripe
  - WePay
  - Amazon
  - Bitbucket

So basically any UI for this has to accommodate 300 zillion auth options. I don't think we need to solve any UX problems here (realistically, installs enable 1-2 auth options and users don't actually face an overwhelming number of choices) but making the login forms less ugly would be nice. No combination of prebuilt elements seems to look very good for this use case.

Test Plan: Registered a new acount with Facebook.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6161