Update to the latest jupyterlab_server and JupyterLab 3.0 packages

[jtpio]

For now we rely on the latest stable jupyterlab_server here:

https://github.com/TheSnakePit/mamba-navigator/blob/ae20cd86635fc44ae5be36fbe71d07f84dbc5709/binder/environment.yml#L8

The idea would be to already start using the 2.0 beta: https://pypi.org/project/jupyterlab-server/2.0.0b7/

Similar idea for the frontend packages define here:

https://github.com/TheSnakePit/mamba-navigator/blob/ae20cd86635fc44ae5be36fbe71d07f84dbc5709/package.json#L36-L41

That could be updated to 3.0.0-beta.2.