Change default URL for Data 8 hub to "/tree" and add entry for JUPYTERHUB_SINGLEUSER_APP

[balajialg]

Changing the default URL from "/retro" to "/tree" based on docs - https://jupyter-notebook.readthedocs.io/en/v7.0.1/migrating/multiple-interfaces.html#nbclassic-and-notebook-7

[ryanlovett]

Looks good.

If you want to run notebook with the new jupyter server app, you would need to install nbclassic and set JUPYTERHUB_SINGLEUSER_APP to jupyter_server.serverapp.ServerApp as well. (as documented)

It might be good to transition away from the legacy notebook server at some point (the app that supports notebook, lab, etc.), as we do in a11y, data100, data101, data102, edx, and stat159.

[balajialg]

Thanks @ryanlovett! nbclassic package is already added to the Data 8 image. I just added the entry for JUPYTERHUB_SINGLEUSER_APP in the config file

[balajialg]

I will merge this to staging for testing purposes and if something is off will revert the changes.