Allow projects to be deleted with ./bin/remove destroy.
Summary: Fixes T5235. Implement PhabricatorDestructableInterface on PhabricatorProject so that projects can be deleted with ./bin/remove destroy.
Test Plan: Created (and then destroyed) a test project. Verified that the corresponding objects (project, slugs and workboard columns) were removed from the database.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Maniphest Tasks: T5235
Differential Revision: https://secure.phabricator.com/D9352