Calculates MD5 hash of static assets on server start. These are appended to the file name's when url_for is used, telling browsers to download the latest version of the files, e.g. style.css?q=81cf0ee.
Inspired by https://github.com/ChrisTM/Flask-CacheBust but uses query strings instead of rewriting the path because fonts inside CSS files break when the path is rewritten.