[WIP] Create route in `VariantController`

Fixes #44

Add to the controller the chance to generate the route with a closure instead of a named route.
This is due to `getResponse` not having the choice to add parameters to the route.

I don't really like this change, maybe we should consider refactoring the helpers in `AbstractAdminController`…
I think it calls for a deep refactor of the helper methods in `AbstractAdminController`.
But I think we can live with that for now if it's working.