Play a sound when receiving a new chat message

Summary:
Ref T7567. Nothing fancy yet, just getting this working. Sound is lightly edited version of "Pop 6":

https://www.freesound.org/people/greenvwbeetle/sounds/244656/

Test Plan: Sent chat, heard sounds.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7567

Differential Revision: https://secure.phabricator.com/D17721