Add support for jupyter-server

[manics]

Proposed change

@fcollonval mentioned in jupyterlab/extension-examples#151 (comment) that Binder doesn't support jupyter-server. Is this something we can add in a backwards compatible manner, so that old versions of Notebook and JupyterLab still launch?

Alternative options

Do nothing and document a workaround.

Who would use this feature?

Anyone using applications based on jupyter-server.

(Optional): Suggest a solution

I don't know what's required to add support (or if it should already be supported but isn't working due to a bug)

[manics]

This will be handled in repo2docker:

• Switch to using jupyter_server to launch singleuser servers repo2docker#956
• Supporting JupyterHub singleuser 3.0.0  repo2docker#1195