Adds the UI dropdown panel

Summary:
Add a dropdown to display notificaitons. Right now
there is nothing real time about it, but we do update the panel
when the user clicks. This panel is only displayed if the
install has notifications enabled and you have them enabled in
your preferences (not using them by default).

Test Plan: Turn off notifications for user1, left them on for user2. Did things from user1 and from user2 on task both were cc'd on. user2 recieved all notifications, user1 recieved nothing. Made new user, made sure everything was switched off by default.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: keebuhm, ddfisher, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2703