Allow users to resign if they have authority over any reviewer

Summary:
Ref T11050. The old rule was "you can only resign if you're a reviewer".

With the new behavior of "resign", the rule should be "you can resign if you're a reviewer, or you have authority over any reviewer". Make it so.

Also fixes T12446. I don't know how to reproduce that but I'm pretty sure this'll fix it?

Test Plan:

• Could not resign from a revision with no authority/reviewer.
• Resigned from a revision with myself as a reviewer.
• Resigned from a revision with a package I owned as a reviewer.
• Could not resign from a revision I had already resigned from.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12446, T11050

Differential Revision: https://secure.phabricator.com/D17558