Migrate away from including auth token within WebSocket tunnel URL

The following HTTP requests example generated by Guacamole client contains authentication service tokens via URL query parameters, which could be leaked from server log files, “Referer header” of HTTP request, etc. 

Example:

GET /workstation/websocket-tunnel?token=<token>&GUAC_DATA_SOURCE=postgresql&GUAC_ID=1&GUAC_TYPE=c&GUAC_WIDTH=1920&GUAC_HEIGHT=1081&GUAC_DPI=96&GUAC_TIMEZONE=Europe%2FBerlin&GUAC_AUDIO=audio%2FL8&GUAC_AUDIO=audio%2FL16&GUAC_IMAGE=image%2Fjpeg&GUAC_IMAGE=image%2Fpng&GUAC_IMAGE=image%2Fwebp

I was able to verify this for both 1.5.2 and 1.5.1, older releases are probably also affected by this.

This is similar to: GUACAMOLE-1775