Add multiple notifications and notification types
Summary:
- Allow more than one notification to be shown.
- Allow notifications to be customized with extra classes.
Test Plan: {F12776}
Reviewers: allenjohnashton, ddfisher, keebuhm, jungejason
Reviewed By: jungejason
CC: aran
Maniphest Tasks: T944
Differential Revision: https://secure.phabricator.com/D2758