Merge pull request #40 from tOkeshu/higher-ping-time

Increase the ping time to 20s
Mute local videos to avoid the huge local echo we have since gum is called for both audio and video at once.
Merge pull request #41 from fqueze/mute-local-video

Mute local videos to avoid the huge local echo we have since gum is call...
Reject calls when already in a call on mobile

Automatically reject calls if we're already in another call on mobile.
Improve the UI to provide a meaningful warning for the caller.
Improve the UI for status messages