Allow to keep the last mute state for a room widget per user

[MTRNord]

Is your feature request related to a problem? Please describe.
When you mute the widget and switch rooms, the mute state isn't kept. This gets annoying sometimes.

Describe the solution you'd like
Some way of saving it on a per widget/room base per user.

Describe alternatives you've considered
Starting muted by default. But that's against the purpose of a conference.

[squahtx]

In #51 we tried storing mute+volume state in localStorage but rejected it because it wouldn't work when third party tracking was disabled and didn't work for popout.

For this use case it may make sense to use localStorage, unless the widget API provides its own storage mechanism.

Remember to test on Firefox: when I tried restoring the volume state too early, the <video> UI displayed the state incorrectly.