fixes link to same object bug

This fixes this bug: https://trello.com/card/link-til-forskellige-materialetyper-f-rer-alle-til-ebogsudgaven/509250ce119c9f1b4400b7eb/6

All the links in material types link to the same object. Now they link to a object if there is only one object of that type else there is a link to a collection.