Restore "Auditor" as an alias for the commit message field "Auditors"
It was recognized by the ad-hoc regular expression which I replaced with a formal parser in D17262.
Allow the former parser to accept "Auditor" as an alias for "Auditors".
Test Plan: Committed a change with Auditor: dog, saw the audit trigger correctly in the web UI.
Reviewed By: chad
Maniphest Tasks: T12197
Differential Revision: https://secure.phabricator.com/D17306