Apply AMAZING DESIGN SKILLS to notification menu

Summary:
I am a fancy designer!

{F12665} {F12666}

Test Plan: Opened/closed menu. Viewed with-notification-count and without-notification count states.

Reviewers: allenjohnashton, ddfisher, keebuhm

Reviewed By: ddfisher

CC: aran, chad, joe

Maniphest Tasks: T974

Differential Revision: https://secure.phabricator.com/D2735