This commit removes the https://www.goatcounter.com/ script that we were using for basic, privacy-friendly analytics. I don't really need that data, so there is not much value in keeping it.
This way, the selected theme is kept when changing pages, and for subsequent visits. To prevent flickering, we set the selected theme in a blocking script directly on load. To speed things up further, we now use a `data-theme` attribute instead of classes on the body tag, this way we don't need to wait until the body element is loaded.
