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