integration with sonata admin

[dbu]

if we have a sonata admin in place, would be cool to integrate with that. one generic way could be to have a controller that accepts the id of any item, show a button in the create toolbar "Edit in Backend" and then have that backend controller try to create an edit url for that "thing", e.g. by loading it from phpcr-odm and redirecting to the corresponding sonata admin class. that should work with orm as well.

if we have anything that is not handled in sonata, we would need a way to identify if the button should be shown or not. maybe the js should just query the backend and show that button if the backend provides the editing url, otherwise not even show the button.