Share notebooks *interactively* between phones.

[switham]

This means, someone on another phone can open notebooks from my phone's server, run the python code (on my server), save changes (on my server), etc.

Distinct from fetching read-only copies of notebooks, [https://github.com//issues/11]

The problem here isn't serving the interactive notebook page (already semi-working), but that this feature needs sessions to prevent people from scrambling each other's work, and logins for security. Also, to implement it one needs to know whether the user came in from this device or another [https://github.com//issues/13]

Not just logins, but a place where I set the permission for other people to run notebooks on my device. Normally one would not want to do this. So, turn the whole feature off and on, and for each notebook, not allowed, allowed for certain people, allowed for everyone (?).