Add a basic notification UI element

Summary: This is extremely basic but dead simple and should cover us for v1, I think. Let me know what features you need.

Test Plan: Used UI example page.

Reviewers: allenjohnashton, ddfisher, keebuhm

Reviewed By: ddfisher

CC: aran, ender

Maniphest Tasks: T944

Differential Revision: https://secure.phabricator.com/D2732