Allow users to unlink their last external account with a warning, instead of preventing the action
Since we now allow email login even if password auth is disabled, it's okay to let users unlink their final account, and it's even reasonable for users to unlink their final account if it is mis-linked.
Just give them a warning that what they're doing is a little sketchy, rather than preventing the workflow.
Test Plan: Unlinked my only login account, got a stern warning instead of a dead end.
Reviewed By: amckinley
Maniphest Tasks: T7732
Differential Revision: https://secure.phabricator.com/D20106