R1:cb6349b88cc2

Implement a "!projects" mail command

Summary: Ref T7199. Implements `!projects` for all objects which implement `PhabricatorProjectInterface`.

Test Plan: Added projects to a task via email.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12246