Rewrite speaker note system

[mgeisler]

The speaker-notes.js file is a little home-grown system for injecting speaker notes to the pages.

I would like to rewrite this to use a more modern development style:

• we should minimize the code when publishing the course
• we should make the label ("Speaker Notes") translatable!
• we could perhaps use a library for some of the state logic?
• we could take advantage of Write mdbook renderer with strong templating system mdbook-i18n-helpers#70 when it's ready to do less on the fly DOM manipulation

[mgeisler]

Making the labels translatable is also being looked at in #1390.