Allow administrators to change usernames

Summary:
Give them a big essay about how it's dangerous, but allow them to do it formally.

Because the username is part of the password salt, users must change their passwords after a username change.

Make password reset links work for already-logged-in-users since there's no reason not to (if you have a reset link, you can log out and use it) and it's much less confusing if you get this email and are already logged in.

Depends on: D2651

Test Plan: Changed a user's username to all kinds of crazy things. Clicked reset links in email. Tried to make invalid/nonsense name changes.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1303

Differential Revision: https://secure.phabricator.com/D2657