Add some basic sound preferences
Summary: Ref T7567. This adds some constants (for adding new sounds), global setting for turning on and off sound (setting) and per thread preference for sound choice. Also specc'd out Mentions, if added.
Test Plan: I tested all the preference wiring, but need to set up notifications locally to verify if this works. Feel free to test.
Reviewed By: epriestley
Subscribers: amckinley, Korvin
Maniphest Tasks: T7567
Differential Revision: https://secure.phabricator.com/D17726