Allow audit comments to be added from Diffusion

Summary:
This is intended to supplant the existing "audit edit" interface. I've changed
them to both drive down the same write pathway, but the UIs are still different.
I'll fully merge them in a future diff.

Add a comment box (like Maniphest and Differential) to Diffusion. When users
make comments, their comments appear on the commit. Any audits triggers they are
responsible for are updated to reflect actions they take, as well.

Currently, audits can only be triggered by packages, but I intend to allow them
to be triggered by users and projects (via herald rules) in an upcoming diff.
Thus some of the language like "projects, users or packages" when the code is
clearly dealing only with "packagePHID".

Test Plan: Made audit updates via commit interface and via existing edit
interface. Verified both interfaces updated correctly, and that audit
responsibility rules were applied properly.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1688