Use Jovyan as the sole user

[diocas]

With the introduction of oAuth tokens (swan-cern/jupyter-extensions#27), we might no longer need to create the user inside the container (to be checked). This would simplify all of our startup scripts, since we can just pre-create the folder structure and put all the config files in the final destination. This would also allow us to simplify the Spawner and just use the normal parameters other Jupyter instances use.

[diocas]

Not a good idea, since users might use some scripts/clients that take the username