Cache static assets

Phew... this was annoying.
I had a bit of trouble with it not working consistently across different browsers, but it *should* be fine now.

This will cache any assets in `www/assets/%version` indefinitely and force revalidation for others. It will also force revalidation for all assets in a dev environment, we wouldn't want to force people to hard reload all the time!

Somebody that knows what they're doing when it comes to HTTP caching (maybe @clone1018?) please take a look, too!