Support subpath URI and token

[oeway]

We serve our xpra client under a subpath(e.g. ws://localhost:8000/desktop) and plus a query string for the authentication token(https://localhost:8000/desktop/?token=s3sdfou239sdofj2o30usdofj23) when initiating the connection. This works with the official client, however, it doesn't seem to work with the osjs xpra application.

This is a link to start a binder server (a free public docker based service): https://mybinder.org/v2/gh/imjoy-team/imjoy-hybrid-desktop/xpra?urlpath=desktop (takes a minute to start up and you will see the official xpra client link).