Mark notifications as read in Differential if we also sent an email

Summary: See D3789. Same thing for Differential.

Test Plan: Created a new revision and made a comment. Verified reviewer got popup notifications but the in-app notifications were delivered already marked as read.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1403

Differential Revision: https://secure.phabricator.com/D3790