Desktop Notification support

Summary:
Fixes T4139. Adds a "Desktop Notifications" panel to settings. For now, we start with "Send Desktop Notifications Too" functionality. We can try to be fancy later and only send desktop notifications if the web app doesn't have focus, etc.

Test Plan:
Made some comments as a test user on a task and got purdy desktop notifications using Chrome. Then did it again with Firefox.

Played around with permissions form with Chrome and got helpful information about what was up. Played around with Firefox and got similar results, except canceling the dialogue didn't invoke my handler code somehow. Oh Firefox!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: rbalik, tycho.tatitscheff, joshuaspence, epriestley, Korvin

Maniphest Tasks: T4139

Differential Revision: https://secure.phabricator.com/D13219