R1:89d1403fe89d

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…