Implement PhabricatorProjectInterface for marking that objects can be tagged with projects

Summary: Ref T2628. This makes Transactions understand objects that can have project relationships, extract project mentions, and handle watching.

Test Plan: See next diff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2628

Differential Revision: https://secure.phabricator.com/D9340