Display Arcanist project in Differential e-mails

Summary:
I need this information quite often.
I don't know how many people are working on a single project and this information will be useless for them but I guess it won't hurt much?

Test Plan: Commented on accepted revision.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2674