Replace old login validation controller with new one

Summary: Ref T1536. We can safely replace the old login validation controller with this new one, and reduce code dplication while we're at it.

Test Plan: Logged in with LDAP, logged in with OAuth, logged in with username/password, did a password reset.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6178