fix(ContactMessages): Use alchemy route proxy

Without that it happens that a controller inheriting from
MessagesController cannot resolve that route correctly.

(cherry picked from commit d845636)

app/controllers/alchemy/messages_controller.rb

@@ -97,7 +97,7 @@ def redirect_to_success_page
       else
         Language.current_root_page.urlname
       end
-      redirect_to show_page_path(
+      redirect_to alchemy.show_page_path(
         urlname: urlname,
         locale: prefix_locale? ? Language.current.code : nil
       )