Add an X-Phabricator-Projects header to outbound mail

Summary: Fixes T4973. For PhabricatorProjectInterface objects, add a header to let clients do mail filtering.

Test Plan: Saw X-Phabricator-Projects: <#goat_farm> in outbound mail.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: rush898, epriestley

Maniphest Tasks: T4973

Differential Revision: https://secure.phabricator.com/D10256