hub: investigate spawning other users UI servers

[oliver-sanders]

For a multi-user setup, if JupyterHub is run under an account with raised privileges then the Hub can spawn a UIS for the authenticated user.

However, JupyterHub does not allow one user to spawn another user's UIS, largely because Jupyter Server is a single-user application, however, the UIS is a multi-user application and this functionality would be highly desirable for us.

We should investigate whether this limitation can be overcome, start points:

• Existing configurations.
• Admin functionality.
• JupyterHub services (these can stop servers, they should be able to start them too).

See also #171 (comment)

[hjoliver]

See also #171 (comment)

Main point from that: you can currently spawn UI Servers on other accounts, but only if you are a Jupyter admin user.

[oliver-sanders]

Thanks to the new(ish) RABC in JupyterHub this is now a configuration problem. Will keep open until we have documented how to configure this.