feat: Minify css

- Add dependency clean-css
- Add do_minifycss function as filter to eleventy
- Add workaround for demo mode with computed data file

source/templates/demo/eleventyComputed.js

@@ -0,0 +1,5 @@
+module.exports = {
+	eleventyComputed: {
+		path: demoMode ? "/" : "/demo/"
+	}
+};