extract searcher_class instantiation into a ContollerHelper module

searcher_class was being instantiated in the same way in home_controller, products_controller, and taxons_controller. This moves that code into a new `Spree::Core::ControllerHelpers::Search` module.

I needed to modify the params used when creating `@searcher` in these 4 classes to add a value saved in the session. It would have been nice to just override once method. Added bonus of cleaning up the common code in these 3 controllers.