Soporte tematizacion de subtipos

You only need to call it once if it's called from the view, but if it was on the controller, you need it on the `new` and `edit`, always, and also on the `create` and `update` in the case that writing fails and they show you again the form...  and I don't like that 4-times-repeated way. I better prefer a decorator, which is only matter of moving it, over the controller; it won't be the first time that someone forgot loading dropdowns data on the failing create/update, breaking them...