Wagonorder: send request to www.apps-bahn.de when availability is unknown

www.apps-bahn.de/wr no longer serves "tryThese" suggestions. So, unless we
know from lib.finalrewind.org/dbdb/has_wagonorder (via ist-wr.noncd.db.de)
that the trais definitely has a wagon order, we send a wagon order request to
www.apps-bahn.de/wr to determine whether it has one or not.

We do cache the result though, so if a user clicks on the wagon order link, we
don't need to request it again.