registration: Query the contract validity

Also, piggy-back a fix of YIN according to what is actually being sent in some other RPC.

Part of #16 (closed) (not as a plugin, because the registration plugin already seems like a good place to put it in).

This is the part that'd be used by foris. There's still a need for the script modifying /etc/config/updater and adding the relevant user list. That'll go into a separate merge request, this is being submitted so it doesn't block others.

Also, the API also provides how many days are left in case the contract is valid. Is it of any use and should I add it here too?