Resolve "Hide editions on view many job offers page if all offers are hidden"

Closes #214 (closed) Problem was: edition.offers contained all offers including hidden ones. However there was an "if hidden don't show" in the frontend which resulted in editions showing without any offers in them. Instead of filtering out editions with no visible offers in the frontend, I changed the backend when getting those offers to only get the non hidden ones. Therefore, editions will now have zero offers if all of them are hidden and therefore not show up. This way also hides the programs which have no visible offers in any of the editions in that program.