Allow notifications to be published as read if the user also is getting an email

Summary: See discussion in T1403. Possibly we'll add a preference for this or something?

Test Plan: Not yet in use. See future diff.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1403

Differential Revision: https://secure.phabricator.com/D3784