Static files are now served with an 8-character hash before the last
extension. For example, `tutti.min.js` is now served as
`tutti.min.abcd1234.js`. When doing a request the hash is removed before
serving the static file.
The hash must be 8 characters long, and is taken from STATIC_FILE_HASH.
It is up to the deployment to change this configuration variable
whenever static files change. This forces browsers that download newly
deployed HTML to also refresh the dependencies (most importantly
JS/CSS).
For this to work, the URL must be built with `url_for('static_xxx',
filename='/path/to/file')`. The 'static' module still returns regular,
hashless URLs.
