Audit - add ability to add user or projects as auditors

Summary: Ref T4687. Trickier part is adding packages; will require some typeahead core changes

Test Plan: add a project as an auditor succuessfully!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4687

Differential Revision: https://secure.phabricator.com/D8704