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