Admin broadcasts (network)

[daniele-athome]

Write an admin script for sending a broadcast message to the whole network.

Allowing general admin status by remote admins isn't an option because we'd be too wide open. We need a special communication component (that we could also use for other tasks in the future, e.g. account migration) that will allow messages from other servers in the network.
Verification method: "from" JID matching a new column in the servers table (e.g. admin_jid) AND digital signature on messages.