Implement "Add to CC" rule for Audits

Summary:
D6660 accidentally allowed you to build Herald rules for commits that take action "Add to CC", but provided no implementation.

Someone at Facebook then wrote such a rule.

Fix forward since there's no real reason not to allow this.

Test Plan: Used `./scripts/repository/reparse.php --herald rXnnnn` to trigger rules. Observed rule trigger and subsequent subscription.

Reviewers: wez, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6845