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