Follow the example of the `<hypothesis-sidebar>` element by making the
`<hypothesis-notebook>` element an unstyled shadow-host which serves as
the root container for the notebook, isolating it from the page's
styles. Inside this is a styled element which serves as the visual
container.

As well as isolating the notebook from the host page's styles, this will
also make it possible to avoid loading annotator styles into the host
page. See https://github.com/hypothesis/client/issues/2979.

To reduce resource usage a little when the notebook is not used, only
the notebook's shadow host is created initially. The styled inner
container is created when the notebook is shown for the first time.