Send old login code to the bottom of the sea

Summary:
Ref T1536. This is extremely reachable and changes the login code to the new stuff.

Notes:

  - I've hard-disabled password registration since I want installs to explicitly flip it on via config if they want it. New installs will get it by default in the future, but old installs shouldn't have their auth options change.
  - Google doesn't let us change the redirect URI, so keep the old one working.
  - We need to keep a bit of LDAP around for now for LDAP import.
  - **Facebook:** This causes substantive changes in what login code is executed.

Test Plan:
  - Logged in / logged out / registered, hit new flows.
  - Logged in with google.
  - Verified no password registration by default.

Reviewers: btrahan, chad

Reviewed By: chad

CC: wez, nh, aran, mbishopim3

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6222