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