Add repository to audit emails
Summary: This is vaguely related to T5791. Add a "REPOSITORY" header to audit emails so that they can be filtered in Gmail.
Test Plan: Commented on an audit and used ./bin/mail show-outbound to inspect outbound email.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: fabe, Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D12997