Improve style of notifications

Summary:

• Gets about 25% of the way toward @chad's notification mocks.
  • YES: Hover states, entire notification is a click target, border, header, footer.
  • NO: Profile pictures (lazy), timestamps (want to refactor time code before introducing a new formatting style), app icons (they'd look funny without timestamps I think)
• Deletes some old files.
• Mostly trying to get this good enough to turn on by default.

Test Plan: Looked at notifications. Clicked some notifications.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4119