Use web components for sidepanel

[brichet]

The side panel currently uses an lumino accordion widget (from the jupyterlab sidebar widget).
Content panels are not correctly displayed when multiple panels are open, some of them with a height of 0px.
This is probably due to the fact that content is updated dynamically: database tables and columns are retrieved only when necessary.

It might be easier, and more accessible, to switch to web components, rather than fixing this implementation.