Cache static files by hash

Implements #2734.

Deployment note: `GRATIPAY_ASSET_URL` needs to be changed to `https://assets.gratipay.com/` on Heroku. I don't think we'll need to change anything on the CDN side, can you confirm that @whit537 ?
And how many bytes are really saved? I don't think we need to care about it here. It is not a technical limitation (such as size of some packet like UDP where you need to squeeze it into), not extra 100$/month for bandwidth, and not performance improvement like SPDY.