Audit - move over to application search

Summary: ...also kills off "PhabricatorAuditCommitQuery" and "PhabricatorAuditQuery", by moving the work to "DiffusionCommitQuery". Generally cleans up some code around the joint on this too. Also provides policies for audit requests, which is basically the policy for the underlying commit. Fixes T4715. (For the TODO I added about files, I just grabbed T4713.)

Test Plan:
Audit: verified the three default views all showed the correct things, including highligthing. did some custom queries and got the correct results.
Diffusion: verified "blame view" still worked. verified paths were highlighted for packages i owned.
Home: verified audit boxes showed up with proper commits w/ audits
bin/audit: played around with it via --dry-run and got the right audits back

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: chad, epriestley, Korvin

Maniphest Tasks: T4715

Differential Revision: https://secure.phabricator.com/D8805