Allow users to turn off desktop notifications

Summary: Fixes T8846. Ref T4103. I just took the shortest reasonable path here, this panel could use some attention on the next Conpherence iteration.

Test Plan: Turned on/off desktop notifications. Observed corresponding behavior in test notifications.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103, T8846

Differential Revision: https://secure.phabricator.com/D16036