Isolate jupyterlab venv extensions

[dfguerrerom]

This PR aims to isolate jupyterlab venv extensions, and therefore make the sepal-contrib apps more robust, since we won't rely on the system installation anymore.

• I also have removed the extra build lines to install jupyterlab which are not longer required from Jupyterlab>3, check Failed to load model class 'MPLCanvasModel' from module 'jupyter-matplotlib' matplotlib/ipympl#460 (comment).
• I have installed my voila branch, and we might keep it until it is fully merged in voila Custom page config hook voila-dashboards/voila#1495.
• I also have removed the notebook_dir param since it is deprecated, changed it to use root_dir
• add jupyter-server-proxy to excluded extensions (b1d3363) due to jupyter-server-proxy labextension creates an error in the console voila-dashboards/voila#1499