When a user makes an audit comment, retroactively trigger an audit

Summary:
If a user comments on a commit but they don't currently have any audits they're
authoritative on, create a new one.

This makes it easier to handle other things more consistently, like figuring out
the overall audit status of a commit and who should get emails.

Test Plan: Made comments on commits I had authority on and did not have
authority on.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, epriestley

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1697