R1:fd0591e16880

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.

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