Merge "Audit" more completely into "Diffusion"

Summary:
Fixes T6630. Long ago, "Audit", "Diffusion" and "Repositories" were three totally separate applications.

This separation isn't useful and the three rapidly became intertwined. Ideally, they would all be one application.

This doesn't take us quite that far, but Audit no longer has any controllers and has little actual behavior.

The "Audit" screen has always just been a SearchEngine view of commits with some filters on it, and this formalizes that and puts a link to it in Diffusion. (This view has other uses, too.)

Test Plan:

• Accessed audit from home page.
• Accessed audit/commits from Diffusion.
• Could no longer uninstall Audit on its own.
• Grepped for /audit/ and AuditApplication.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6630

Differential Revision: https://secure.phabricator.com/D17186