Make authorities post authority_certificate to other authorities
At the moment, offline authorities work, until their certificates expire. They post their votes to other authorities, and fetch votes from other authorities, and no-one needs to connect to them.
But when their certificates expire, no-one can get their new certificate, because it's only available via a download from that authority.
(There's a hack that involves placing the new certificate in another authorities cached certificates file. Which is not ok on the public network.)
Let's make a way for offline authorities to post their certificates to other authorities, and make all authorities do it when they have a new certificate.