Mute self / turn off camera

[janlelis]

• A user should be able to mute themself (or turn off their video stream)
• Important: This must change the actual peer session, so that the media does not get sent

[janlelis]

As noted in https://github.com/palavatv/palava/issues/42#issuecomment-632695533, muting already works, so the following steps need to be done to implement this:

• [x] Reactivate control knobs: https://github.com/palavatv/palava-web/blob/474e28b3184cc72c97e958aed638bcebc707f6ec/src/components/Party.vue#L55-L81
• [x] Only show mute knob is local stream has audio (same with video)
• [x] Track own mute / video-sending state in the party components' state (data:), both knobs should let you toggle these states, see https://github.com/palavatv/palava-web/blob/474e28b3184cc72c97e958aed638bcebc707f6ec/src/components/Peer.vue#L122-L132 for inspriation (also note that the svgs for both states are already available in the assets folder)
• [x] Actual mute functionality to be triggered is here: https://github.com/palavatv/palava-client/blob/master/coffee/local_peer.coffee#L70
• [x] Put video on hold still needs to be implemented in the client library