Update batchspawner-singleuser for jupyterlab 3.0 ?

This JupyterHub issue talks about a re-implementation or update of the singleuser server:
* https://github.com/jupyterhub/jupyterhub/pull/3128 (+ things linked from there)

I think this will probably affect us with the batchspawner wrapper, but I'm not entirely sure how. I think we need to check on things at least and can possibly make things simpler (?)

@cmd-ntrf - I seem to remember you did something about the current batchspawner-singleuser. Is this in your realm of knowledge?