Create a simple server for the navigator

[fcollonval]

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Description

Currently the navigator is built on top of a JupyterLab server. This is not a good idea as it brings endpoints for kernels, content management, terminal,... . We should build on top of a simple server.

[jtpio]

A simple server should indeed be enough :+1:

There were some ideas of being able to build environments using the navigator in a visual manner, but this will probably not require kernels or terminals.