Implement new auth login flow and login validation controller

Summary:
Ref T1536. None of this code is reachable.

Implements new-auth login (so you can actually login) and login validation (which checks that cookies were set correctly).

Test Plan: Manually enabled FB auth, went through the auth flow to login/logout. Manually hit most of the validation errors.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6162