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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978

Differential Revision: https://secure.phabricator.com/D17187