R1:92678eb0504b

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…