Make Jupytext's server extension compatible with Jupyter Server based apps like Voila

[mwouts]

The use case for this was reported at voila-dashboards/voila#107 by @SylvainCorlay. Voila is an application that can render notebooks as interactive web pages. Since voila reexecutes the notebook in full before serving it, there's no point in preserving the outputs. So it would be nice to store these notebooks in the form of Jupytext files.

Voila is internally based on jupyter_server rather than on notebook. In particular the default contents manager is LargeFileManager from jupyter_server.services.contents.largefilemanager rather than from notebook.services.contents.largefilemanager. Jupytext's extension should be aware of then at the time of substituing the default CM with Jupytext's one.

Also, we should add a jupyter_server_config.d/jupytext.json file that activates Jupytext's server extension on the ServerApp. That way, using Jupytext with Voila would simply be a matter of having both programs installed.

[mwouts]

With the latest commit we do see Jupytext's contents manager class being created when jupyter server is started (with no other config than having Jupytext installed):

(python3) C:\Users\Marc\Documents\GitHub\jupytext>jupyter server
[I 21:12:47.182 ServerApp] [Jupytext Server Extension] Deriving a JupytextContentsManager from LargeFileManager
[I 21:12:47.183 ServerApp] Serving notebooks from local directory: C:\Users\Marc\Documents\GitHub\jupytext
[I 21:12:47.183 ServerApp] The Jupyter Server is running at:
[I 21:12:47.183 ServerApp] http://localhost:8888/?token=...