loadCSS was used as a polyfill for rel=preload but it seems we only used
it on a single CSS file which did not help much. Changed the icon CSS to
a blocking load and preload the woff2 variants which does work in modern
browsers (Firefox currently needs the network.preload about:config
setting but should gain proper support soon).
Co-authored-by: zeripath <art27@cantab.net>