Load highlight.js from a CDN

[mojavelinux]

Load highlight.js from a CDN like we do with the HTML5 output instead of using the bundled version. Of course, we can still recognize the property of highlightjs dir so that it's possible to use the bundled version, but we should be consistent with the HTML5 output by default.

[ggrossetie]

I agree the version in the reveal.js plugin is outdated, see: hakimel/reveal.js#2562

We should use a copy of https://github.com/hakimel/reveal.js/blob/master/plugin/highlight/highlight.js without highlight.js.

@obilodeau What do you think? The downside is that we will need to sync this file when we upgrade reveal.js.

[ggrossetie]

Not sure about the CDN part, considering that we are using a local copy of reveal.js but otherwise 👍

[obilodeau]

I was never particularly affected by this so don't have a big incentive for this. That said, I don't mind merging it. Just make sure changes to the release process are documented. Like looking at upstream (both reveal.js' integration and highlight.js) and merging back before releases, etc.

Edit: my previous wording wrongly reflected that I didn't really care