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