show warning when a new invoice can be generated

app/views/spree/admin/invoices/index.html.haml

@@ -1,3 +1,7 @@
+- if show_generate_invoice_button?(@order)
+  .alert-box.warning 
+    =t(:order_has_changed_and_invoice_might_not_be_up_to_date)
+
 = render partial: 'spree/admin/shared/order_page_title'
 = render partial: 'spree/admin/shared/order_tabs', locals: { current: 'Invoices' }
 

config/locales/en.yml

@@ -4182,6 +4182,9 @@ See the %{link} to find out more about %{sitename}'s features and to start using
           line_item_adjustments: "Line Item Adjustments"
           order_adjustments: "Order Adjustments"
           order_total: "Order Total"
+      invoices:
+        index:
+          order_has_changed_and_invoice_might_not_be_up_to_date: "The order has changed since the last invoice update. The invoice shown here might not be up-to-date anymore."
       overview:
         enterprises_header:
           ofn_with_tip: Enterprises are Producers and/or Hubs and are the basic unit of organisation within the Open Food Network.

spec/system/admin/order_spec.rb

@@ -1102,12 +1102,14 @@ describe '
             expect(page).to have_content "#{customer.first_name} #{customer.last_name} -"
             expect(page.find("table").text).to have_content(table_header)
 
-            # the New invoice button should be visible
+            # the New invoice button + the warning should be visible
             expect(page).to have_link "Create or Update Invoice"
+            expect(page).to have_content "The order has changed since the last invoice update. The invoice shown here might not be up-to-date anymore."
             click_link "Create or Update Invoice"
 
             # and disappear after clicking
             expect(page).to have_no_link "Create or Update Invoice"
+            expect(page).to_not have_content "The order has changed since the last invoice update. The invoice shown here might not be up-to-date anymore."
 
             # creating an invoice, displays a second row
             expect(page.find("table").text).to have_content(table_contents)