Add an "Auditors" rule for Commits

Summary: Fixes T5889. You can't write a rule like "if no other Herald rules did anything...", but you can use this rule to check for Owners or an explicit "Auditors" field doing things.

Test Plan: Using the test console, ran an "Auditors" rule against a commit with and without an auditor. Got expected pass/fail outcomes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5889

Differential Revision: https://secure.phabricator.com/D17221