Cleanup and Optimize

Here are some things which can be done to improve page load speed:

* [x] Caching: use caching with an `ETag` header. (was broken)
* [ ] Images: convert all image assets, except avatar, to `svg` or `webp`.
* [ ] Split CSS Files: I know that downloading all CSS files in one go is a great idea but it is not worth the time when 72% of the rules are not used on the current page. Here a great article about the matter. <https://www.igvita.com/2012/06/14/debunking-responsive-css-performance-myths/>