Properly limit per-quantity calculator options

item-adjustments and quantity-adjustments have their own config for
allowed calculators. When presenting a list of calculators for admins
editing promos we need to respect that config.

backend/app/views/spree/admin/promotions/actions/_create_quantity_adjustments.html.erb

@@ -4,5 +4,8 @@
   <%= number_field_tag field_name, promotion_action.preferred_group_size, class: "fullwidth", min: 1 %>
 </div>
 
-<%= render "spree/admin/promotions/actions/create_item_adjustments",
-  promotion_action: promotion_action, param_prefix: param_prefix %>
+<%= render "spree/admin/promotions/actions/promotion_calculators_with_custom_fields",
+  calculators: Rails.application.config.spree.calculators.promotion_actions_create_quantity_adjustments,
+  promotion_action: promotion_action,
+  param_prefix: param_prefix
+%>