R1:97ea6ea619d1

Add a basic first-class audit UI

Summary:
Currently, audits are only accessible through the Owners tool. Start moving them
to their own first-class tool in preparation for broader audit integration.

- Lay some infrastructure groundwork (e.g. AuditQuery).
- Build a basic /audit/ view.
- Show audits on the commit page in Diffusion.

This has some code duplication with stuff we've already got, but I'll merge
everything together as we move forward on this.

Test Plan: Looked at /audit/ and a commit.