Send mail to audit comment author too

Summary: Previously, you would not receive a mail message for the first comment you make on an audit, but you would for subsequent comments because everyone who's made a comment would be CCed on the email. This mirrors DifferentialTransactionEditor's getMailTo which always adds `$object->getAuthorPHID()`.

Test Plan: With self mail turned on, made the first comment on a commit and received an email for it. With self mail turned off, commented on a different commit and saw in `bin/mail list-outbound` that the message was voided.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8650