Raise error when attaching item without dialog to a catalog

I've added a validation function into the ServiceTemplate model, it fixes the bug when editing a catalog item.
When attaching items to a ServiceTemplateCatalog, an exception is raised, and it's rescued in the controller.
The obsolete disabled order button for dialog-less items was also removed.

https://bugzilla.redhat.com/show_bug.cgi?id=1145266