R1:e0087a9f7a07

Add JX.Leader: synchronization over localStorage

Summary:
Ref T6559. Adds a "JX.Leader" primitive to replace the synchronization over Flash. This does a couple of things:

- Offers an "onBecomeLeader" event, which we can use to open the WebSocket.
- Offers an "onReceiveMessage" event, which we can use to dispatch notifications and handle requests to play sounds and send desktop notifications.
- Offers a "broadcast()" method, which we can use to send desktop notification and sound requests.

Test…