[WIP] Improve vimpatch report styling.

See https://fwalch.github.io/doc/build-reports/vimpatch/. Footer is now below the body; I also did a small refactoring to have the PR section in the HTML template as well.
Up to you, I would punt on it for now. There's a lot of tweaks that I would like to make to the website in the near future which we can handle in another PR.