Import LargeFileManager from jupyter_server if available

[jtpio]

Attempt at fixing the issue noticed in jupyter/notebook#6315 (comment).

The next major version of Notebook (v7) will be based off JupyterLab components and will be running on a Jupyter Server: https://jupyter.org/enhancement-proposals/79-notebook-v7/notebook-v7.html

This is already the case on the main branch in the Notebook repo: https://github.com/jupyter/notebook. The first pre-release is out so it can also be installed with pip install --pre -U notebook.

The new notebook package doesn't expose LargeFileManager since it relies on jupyter_server now. This PR proposes to support multiple cases of users:

• running Notebook v6 only (on the Classic Notebook Server)
• running JupyterLab (on Jupyter Server) only
• running Notebook v7 pre-release (on Jupyter Server)

[mwouts]

Hi @jtpio , thank you for your PR! Oh I see that the transition to jupyter_server is finally happening, that's great. I think I'd like to put the new import at the top of that paragraph, since it will become the defaut case for modern installations, don't you think so?

[jtpio]

Thanks @mwouts for the review!

That sounds good to me. jupyter_server is indeed the way forward and relying on notebook for server extensions is being phased out progressively.

I see you opened #934 as a continuation, so we can close this PR and continue there on #934 if you want. Thanks!

[jtpio]

Closed in favor of #934