This fixes an issue where the user muted by focus is able to unmute themselves even when the sender limit has been reached.

* feat(conference) Impl audio/video mute disable when sender limit is reached.
Jicofo sends a presence when the audio/video sender limit is reached in the conference. The client can then proceed to disable the audio and video mute buttons when this occurs.

* squash: use a different action type and show notification.