Audit - add ability to add a package as an auditor

Summary: Fixes T4687. This was also pretty easy...!

Test Plan: made a package with a test user as owner. added package as owner. looked right on commit page. logged in as test user and verified audit showed up on home page.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: chad, epriestley, Korvin

Maniphest Tasks: T4687

Differential Revision: https://secure.phabricator.com/D8705