Restore "Auditor" as an alias for the commit message field "Auditors"

Summary:
Fixes T12197. I think this field was never recognized by Differential (it doesn't appear in D17070, but maybe that isn't the right change).

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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12197

Differential Revision: https://secure.phabricator.com/D17306