Mark Maniphest notifications read if we send the user an email

Summary:
See D3784, T1403. When we send a user an email and a notification from Maniphest, mark the notification as read.

(It would be nice to do the thing with multiplexMail() a little less hackily, but it gets very complicated to do correctly because we require handles but sometimes do not have an actor/user so I'm punting for now.)

Test Plan: Acted on a task, verified notification was marked read because I received an email.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1403

Differential Revision: https://secure.phabricator.com/D3789