Notebook class creates the container element and instantiates the preact
component NotebookModal.

NotebookModal listens to the openNotebook event sent through the
eventBus and hides and shows the modal accordingly.

The creation of the iframe is a costly operation: it involves several
backend calls and the rendering of it in the DOM. Therefore, we delay
the creation of the iframe element until the NotebookModal listens for
the `openNotebook` event.

The useEffect's cleanup of the NotebookModal componente has significant
effects because it resets the overflow CSS property of document.body.

As pointed by @robertknight, `render(null, container)` destroys the
component triggering the useEffect's cleanup function.