Prevent users from resigning from audits they've already resigned from
Summary: Ref T10978. Since "Resigned" is a status in Audit, you could repeatedly resign. This is confusing; prevent it.
Test Plan: Tried to resign twice; was only allowed to resign once.
Reviewed By: chad
Maniphest Tasks: T10978
Differential Revision: https://secure.phabricator.com/D17187