[Enhancement] Implement the websocket fallback as a real fallback

What problem is solved by the new feature Currently, the websocket fallback (enabled via WS_FALLBACK=true) is only used when a device cannot use window.RTCPeerConnection. Some admins have expected it to be a fallback to the WebRTC connection, so it is also used whenever the WebRTC connection fails.

Describe the feature If the WebRTC connection fails (3 times?) the websockets fallback is used instead.

Additional context See also #215 Only implement this after implementing traffic encryption for the websocket fallback (part of #180)