Mute/unmute the sound in the web interface

[nirs]

Some people (e.g. me) do not like the soundtrack. It will be useful to mute the sound easily from the web UI without muting the entire machine.

The UI should probably be at the top of the window near the other elements, something like a laptop mute button maybe.

The system should keep the mute state; closing and opening the browser will keep the sound muted.

I think html5 local storage api is the best way to implement this, since we don't have any concept of authentication in this application.
See https://html.spec.whatwg.org/multipage/webstorage.html#storage-2