This implements a system for speaker notes via `details` elements and
some JavaScript. The general idea is
1. You add speaker notes to each page by wrapping some Markdown code
in `<details> … </details>`. This is a standard HTML element for,
well extra details. Browsers will render the element with a toggle
control for showing/hiding the content.
2. We inject JavaScript on every page which finds these speaker note
elements. They’re styled slightly and we keep their open/closed
state in a browser local storage. This ensures that you can keep
them open/closed across page loads.
3. We add a link to the speaker notes which will open in a new tab.
The URL is amended with `#speaker-notes-open`, which we detect in
the new tab: we hide the other content in this case.
Simultaneously, we hide the speaker notes in the original window.
4. When navigating to a new page, we signal this to the other window.
We then navigate to the same page. The logic above kicks in and
hides the right part of the content. This lets the users page
through the course using either the regular window or the speaker
notes — the result is the same and both windows stay in sync.
Tested in both Chrome and Firefox. When using a popup speaker note
window, the content loads more smoothly in Chrome, but it still works
fine in Firefox.
Fixes#53.
