Excluded authored commits from "Ready to Audit"; handle unreachable commits better

Summary:
Ref T10978. I'm inching toward cleaning up our audit state. Two issues are:

• Authored commits show up in "Ready to Audit", but should not.
• Unreachable commits (like that stacked of unsquashed stuff) show up too, but we don't really care about them.

Kick authored stuff out of the "Ready to Audit" bucket and hide unreachable commits by default, with constraints for filtering. Also give them a closed/disabled/strikethru style.

Test Plan:

• Viewed audit buckets.
• Searched for reachable/unreachable commits.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978

Differential Revision: https://secure.phabricator.com/D17279