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 ?
@pjz But since Aspen dispatches to `gratipay.css` before `gratipay.css.spt`, unless we unlink the more specific file we'll just get the old content instead of the new.