Audit / Herald - make audits added via herald "required"

Summary: as opposed to "requested". Also re-jigger how the "reason" works so the herald editor can get more specific data rather than a generic message. Fixes T6345 along with companion diff D10726.

Test Plan: made a herald rule to add auditors to a commit and saw it work!

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: Korvin, epriestley

Maniphest Tasks: T6345

Differential Revision: https://secure.phabricator.com/D10730