Show cannot add item to shipped order in the Add products section

app/models/spree/order.rb

@@ -643,10 +643,6 @@ module Spree
       voucher.rate(self)
     end
 
-    def can_modify?
-      !shipped? && !canceled?
-    end
-
     private
 
     def reapply_tax_on_changed_address

app/views/spree/admin/orders/_add_product.html.haml

@@ -6,6 +6,8 @@
 
     - if @order.canceled?
       = t(".cannot_add_item_to_canceled_order")
+    - elsif @order.shipped?
+      = t(".cannot_add_item_to_shipped_order")
     - else
       .field.nine.columns.alpha
         = label_tag :add_variant_id, Spree.t(:name_or_sku)

app/views/spree/admin/orders/_shipment_manifest.html.haml

@@ -18,7 +18,7 @@
         = line_item_shipment_price(line_item, item.quantity)
 
       %td.cart-item-delete.actions
-        - if shipment.can_modify? && can?(:update, shipment) && shipment.can_modify?
+        - if shipment.can_modify? && can?(:update, shipment)
           = link_to '', '#', :class => 'save-item icon_link icon-ok no-text with-tip', :data => {'shipment-number' => shipment.number, 'variant-id' => item.variant.id, :action => 'save'}, :title => t('actions.save'), :style => 'display: none'
           = link_to '', '#', :class => 'cancel-item icon_link icon-cancel no-text with-tip', :data => {:action => 'cancel'}, :title => t('actions.cancel'), :style => 'display: none'
           = link_to '', '#', :class => 'edit-item icon_link icon-edit no-text with-tip', :data => {:action => 'edit'}, :title => t('actions.edit')

app/views/spree/admin/orders/edit.html.haml

@@ -24,7 +24,7 @@
   = admin_inject_order_cycles(@order_cycles)
   %div{"ng-controller" => "orderCtrl", "ofn-distributor-id" => @order.distributor_id, "ofn-order-cycle-id" => @order.order_cycle_id}
 
-    = render :partial => 'add_product' if can?(:update, @order) && @order.can_modify?
+    = render :partial => 'add_product' if can?(:update, @order)
 
     - if @order.line_items.empty?
       .no-objects-found

config/locales/en.yml

@@ -4441,6 +4441,7 @@ See the %{link} to find out more about %{sitename}'s features and to start using
       orders:
         add_product:
           cannot_add_item_to_canceled_order: "Cannot add item to canceled order"
+          cannot_add_item_to_shipped_order: "Cannot add item to shipped order"
           include_out_of_stock_variants: "Include variants with no available stock"
         shipment:
           mark_as_shipped_message_html: "This will mark the order as Shipped.<br />Are you sure you want to proceed?"