Explicitly decline to add commit authors as auditors from Herald

Summary:
Fixes T12304. If you have a Herald rule which tries to add a commit author as an auditor, it fails validation when trying to apply.

Stop trying to apply these transactions, and explicitly tell the user why. Differential already uses a similar ruleset around reviewers, but Audit was using older code.

Test Plan:

• Wrote a Herald rule to add A, B and C as auditors.
• Committed as A.
• After change, saw B and C added with transacript guidance that A was the author.

{F3235660}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12304

Differential Revision: https://secure.phabricator.com/D17404