Don't include resigned reviewers in the Differential "To" list

Summary: Ref T12689. See PHI178. This isn't a complete solution (you may still get mailed via packages/projects) but should fix the obvious issue, where "Resigned" reviewers are incorrectly always sent mail directly.

Test Plan: Had Alice resign, interacted as Bailey, no mail to Alice.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12689

Differential Revision: https://secure.phabricator.com/D18758