Improve far-future support: generate dynamically generated files automatically (f.e. image style derivatives)

Problem/Motivation

There's a bunch of issues related to image styles and the CDN module. And in combination with cdn_cloudfront_private it allows using fully private CDN.

Proposed resolution

Add image style generation to FarFuture feature, it allows storing image derivatives with external CDN.

Remaining tasks

Code review and testing.

User interface changes

Nothing, working out of the box.