Make jupyterlab_server optional

[davidbrochart]

Problem

JupyterLab currently depends on jupyterlab server. For alternative servers to the JupyterLab frontend, such as jupyverse, this dependency is really not necessary, and makes the installation more problematic.

Proposed Solution

jupyterlab_server should either be an optional dependency, that would be installed e.g. with:

pip install jupyterlab[jupyter_server]  # would install jupyterlab_server

or we could keep it as the default server (i.e. jupyterlab_server would be installed when pip install jupyterlab), but it wouldn't be installed when another server is specified:

pip install jupyterlab[jupyverse]  # would not install jupyterlab_server, but would install jupyverse instead

[jtpio]

Thanks @davidbrochart for opening this 👍

It would indeed be great if JupyterLab could work without the full set of endpoints exposed by jupyterlab_server. This would also make it easier to build custom distributions (both server side with jupyverse and client side).

Linking to jupyterlab/jupyterlab_server#167 as somehow related.

Adding to the 4.0 milestone for consideration as this might bring some breaking changes.

[linlol]

How about also make a traitlet config to select jupyter _server/jupyverse

[krassowski]

Clearing the milestone as the associated PR was closed #12625 (comment), we can revisit if we find a backward compatible way to solve this.

Maybe jupyter-server/team-compass#11 could be the ultimate solution here.