Adds a button to the existing top-right button group on chapter and
section pages that hides everything outside of the breadcrumb, the
heading, the rendered questions and the link to the next section or
chapter at the end of the page. The state is persisted via
localStorage so it survives navigation. On pages without questions a
small notice is shown instead.
Replaces the previously published userscript by integrating the same
behaviour directly into the site.
