Add a "problem commits" prebuilt filter to Audits

Summary: Ref T4715. We show this number on the homepage, provide an easy way to query matching commits.

Test Plan: Clicked "problem commits", saw them.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4715

Differential Revision: https://secure.phabricator.com/D8880