Allow more granularity on real-time notifications

Summary: Fixes T12792. Expands the Notifications to "web, desktop, both, or none" for real-time notifications in settings.

Test Plan: Test with "test notifications" button, and while logged into two accounts with each of the 4 settings.

Reviewers: epriestley

Reviewed By: epriestley

Spies: Korvin

Maniphest Tasks: T12792

Differential Revision: https://secure.phabricator.com/D18457