upgrade repository delete function to full-blown workflow

Summary: fancy title. really just make the delete() method aware of related objects and build a quick workflow which calls delete(). also make commit delete savvy about audit requests.

Test Plan: deleted a repository per the instructions given to me in the web UI

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1416, T1958, T1372

Differential Revision: https://secure.phabricator.com/D3822