Show expanded speaker notes on print page

[mgeisler]

The /print.html page is concatenates all pages in the book and can be reached from the printer icon in the top-right corner.

On this page, we will have many speaker notes. Since the page is for people who want all information in one place, I think it makes sense to render all these notes in their expanded form.

I wrote the logic for this as a separate little function since it has to be different: we don’t want to add an id attribute to the generated headers (since there will be more than one of them) and we don’t want to generate links to open the notes into a separate window.