MultipleObjectsReturned error thrown if display_key is empty in success view

[MarkusH]

[ZoltanOnody]

Currently we generate the display_key only if the Allow display checkbox is checked. I suggest to change this behavior. I'd generate the display_key anyway. It would have a couple of advantages.

1. If the admin decides to allow display in the future, he would be able to search for /show/... links in the database.
2. We will avoid this error.

Of course this implementation would require to change the /show/... link behavior. Currently it's possible to show the data via a link also if the admin already disallowed it for the form.