Hydrate ipywidget and anywidget from state on page load

[stevejpurves]

When a myst site loads we currently don't display ipywidgets and actually strip out the default text/plain and widgets mimebundle, swapping it out with a html placeholder fragment.

This is because we can't display ipywidgets without a kernel, as myst doesn't process the widget state and ensure it is a part of the built content.

Aim of this issue is to track progress on changing this and getting ipywidgets to hydrate on load

TODO:

• extend thebe to include a new WidgetManager that can be used without a kernel (passive thebe manager thebe#756).
• add more passive rendering to myst-theme to use the WidgetManager and ensure that widgets are hydrated from state (Rehydrate Widget State #426) from @dwootton 💪.
  • 🐞 widgets do not render on SSR load / page refresh (to reproduce, navigate to a page with widgets, then refresh the page - a "widget model not found" will be displayed, but the same page works fine when navigating to the page)
• (mystmd) extend mystmd to process the widget state at notebook.metadata.widgets and get this onto the built .json (💦 building widget state mystmd#1382)
  • (mystmd) make any widgets state information that we build "out-of-band" so that it does not affect initial page load (ping @fwkoch)
  • (myst-theme) handle loading the out-of-band widget state
• (myst-theme) display a small message when a user interacts with a widget that has no kernel
  • start the compute server (if compute is enabled) when a widget is touched
• (myst-theme) there are timing issues because of the current way we render outputs
• (myst-theme) support tlfdraw / anywidgets - styles no loading?