Split the index.html into smaller, cachable, parts

[Aran-Fey]

Currently, our JS and CSS is inlined in the index.html, so browsers are forced to re-download it every time. We should extract these into static files that the browser only has to load once and can then cache.

Bonus: Most of our file size seems to be coming from highlightjs. We should try to find a way to lazy-load it on demand.