handle vanished zones in powerdns

[luto]

Zones may not be present in the PowerDNS API when we expect them to. This may happen if some other system changed the data or when PowerDNS temporarily returns no or faulty data.

If we detect that a zone is present in our database, but not in API responses, present the user with a "this zone has vanished, want to delete?" screen. In case the user does not have permission to delete that zone, present a similar screen, asking them to contact their admin.