Adding a UI to let users share their servers to other users and groups

[Hyrla]

Hello,
I've added the necessary code related to my issue #437.

It replaces the default link share UI in cases where Jupyter is running behind a Hub that allows the creation of shares. It enables users to:

• Create a share to the current server for a user or a group using a search box.
• Revoke a share for a user or a group.

The permissions needed (both in Hub and Lab) are:

• "shares!user" to manage users' shares.
• "read:users:name" to create shares by designating a user by their name (TODO: check if it's really necessary if list:users is already in scope?).
• "list:users" (not mandatory) to list users for the search box.
• "list:groups" (not mandatory) to list groups for the search box.
• "servers!user" (not mandatory) to allow other users to start and stop the shared server

My code does not:

• Work if neither list:users nor list:groups is in scope. I wanted to add an option to invite someone by typing their exact username, without having to search.
• Manage share codes, which may be useful in some scenarios.

This development has been made for my own internal purposes, but I think it could be useful to everyone. Comments and critiques are welcome!

[github-actions]

👈 Launch a Binder on branch Hyrla/jupyter-collaboration/main