Add an administrative tool for deleting users

Summary:
Allow administrators to delete accounts if they jump through enough hoops.

Also remove bogus caption about usernames being uneditable since we let admins edit those too now.

Test Plan: Tried to delete myself. Deleted a non-myself user.

Reviewers: csilvers, vrana

Reviewed By: csilvers

CC: aran

Maniphest Tasks: T1184

Differential Revision: https://secure.phabricator.com/D2767