Allow add_user.php without password auth

Summary: Fixes T1527.

Test Plan:
Generated e-mail login URL.
Enabled password auth, visited the URL, saw Change Password.
Disabled password auth, enabled Facebook auth, visited the URL, saw Link Facebook account.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1527

Differential Revision: https://secure.phabricator.com/D6006