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