R1:1e3cd8afa839

Improve notification list view

Summary:
- Provide a filter to show just unread notifications.
- Visually show which notifications are unread.
- Style tweaks to make notifications more readable.

Test Plan: {F13494}

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2883