Add a "Needs Verification" state to Audit

Summary:
Fixes T2393. This allows authors to explicitly say "I think I fixed everything, please accept my commit now thank you".

Also improves behavior of "re-accept" and "re-reject" after new auditors you have authority over get added.

Test Plan:

• Kicked a commit back and forth between an author and auditor by alternately using "Request Verification" and "Raise Concern".
• Verified it showed up properly in bucketing for both users.
• Accepted, added a project, accepted again (works now; didn't before).
• Audited on behalf of projects / packages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2393

Differential Revision: https://secure.phabricator.com/D17252