Audit - add mail tags

Summary: Fixes T2497. I'm not sure where we are with subscribers and custom vs normal codepath, but the mailtags implementation makes no assumptions and can handle it either way.

Test Plan: made a commit and got some sensible mail tags

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T2497

Differential Revision: https://secure.phabricator.com/D10712