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 ?
Yikes. So we're converting `gratipay.css.spt` and `gratipay.js.spt` to `gratipay.css` and `gratipay.js` files, eh?