R1:083cd5fd9d69

Add a "Mark All Read" button for notifications

Summary: Adds a button to mark everything as read.

Test Plan: Clicked button.

Reviewers: btrahan, jungejason, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T974

Differential Revision: https://secure.phabricator.com/D2812