Conpherence - add dropdown menu

Summary: Fixes T3641. Probably needs some @chad love though on colors and what have you. Technique was to jam this into the existing notifications stuff as much as possible. I think its "okay" but if we were to add more stuff here (like a 3rd application) this could get a quality pass to consolidate even more code.

Test Plan: played with it in Chrome and Safari - looks reasonable

Reviewers: chad, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3641

Differential Revision: https://secure.phabricator.com/D6708