Show Structure mode for staff users

Currently staff users does not have access to the structure mode in a apphooked model if they don't have permission on the page the apphook is attached to.
This is somewhat limiting as you may want a user to be able to change a model without having permission on the page.
This change shows the structure / content buttons disregarding the real permissions of the user on the said model, which are enforced anyway by the model admin / placeholder admin