Wrap form element in a form

This makes it easier to control in the next commit.

app/views/admin/products_v3/_sort.html.haml

@@ -4,6 +4,6 @@
     - if search_term.present? || producer_id.present? || category_id.present?
       %a{ href: "#", class: "button disruptive", data: { reflex: "click->products#clear_search" } }
         = t(".pagination.clear_search")  
-  %div.with-dropdown
+  %form.with-dropdown
     = t(".pagination.per_page.show")
     = select_tag :per_page, options_for_select([15, 25, 50, 100].collect{|i| [t('.pagination.per_page.per_page', num: i), i]}, pagy.items), data: { reflex: "change->products#change_per_page" }