Use JupyterHub for teaching purposes

[LauWien]

As suggested by @volker-krey, we can think about setting up a JupyterHub for teaching purposes. With JupyterHub, the installation of message_ix would be obsolete. Every user in the training could simply log into the hub and has directly the latest and correct environment on hand to run and directly dive into the MESSAGEix tutorials in Jupyter Notebooks.

[khaeru]

This would depend on/or essentially be achieved by #490.