Set-up live collaboration

[erwanp]

Radis-Lab links can already be shared (distant user access the latest saved version)
but adding the --collaborative option should allow real-time collaboration :

• https://jupyterlab.readthedocs.io/en/stable/user/rtc.html

Also :

• add https://github.com/jupyterlab-contrib/jupyterlab-link-share extension
• remove "Share Binder" link from menu bar, which starts a mybinder.org instance (and not gesis).

[erwanp]

@arunavabasu-03 if interested