Update reveal.js version

[Andrea Ferretti]

The included Reveal.js version is quite outdated. I tried using a directive like

Reveal Url: https://cdn.jsdelivr.net/npm/reveal.js@4.1.3/js/reveal.js

but this has two drawbacks:

• first, it is better to just include the relevant files into the HTML, so that we obtain a self contained file
• second, it did not really work, apparently because the page tries to download the JS and CSS files with the wrong MIME type

Would it be possible to bundle a more up to date version of Reveal? Among other things, this would fix an issue in the ?print-pdf version, which inserts additional blank pages