The old flavored consensus URL format made it harder to decode URLs
based on their prefixes, and didn't take into account our "only give
it to me if it's signed by enough authorities" stuff.
The point of doing SHA256 twice is, generally, is to prevent message
extension attacks where an attacker who knows H(A) can calculate
H(A|B). But for attaching a signature to a document, the attacker
already _knows_ A, so trying to keep them from calculating H(A|B) is
pointless.
