Show commits in /audit/

Summary:
The general idea here is to build a Differential-like dashboard which shows all
the things you need to audit and all the things that other people have raised
issues with, so you have a one-stop "what do I need to deal with?" interface.

• Add problem commits to the "active" view of /audit/.
• Add problem commits to homepage.
• Add commit browsing interfaces to /audit/.
• Add an "Audit" app button.

Test Plan: Looked at homepage, commit filters. Audited commits, verified state
changes reflected properly.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1712