Make the users toolbar item generic

[brichet]

The users toolbar item in the document toolbar is already reused in jupytergis.

It would also be useful in jupyter-chat, and possibly in other shared documents.

We were wondering with @martinRenou about making it more generic and moving it to jupyter collaboration, to let any extension include it in its toolbar.

By making it more generic I'm thinking about:

• handle any shared document
• add a registry to register how users are rendered (with a default to the avatar/intitial without action on click)

[martinRenou]

We could think of having it in the Notebook viewer too, and scroll down to the collaborator position upon click.