Add expected_amount to Invoice

expected_amount returns the sum of all associated orders together with
the transport costs.

app/views/finance/invoices/show.html.haml

@@ -29,10 +29,14 @@
         %dd><
           - @invoice.orders.order(:ends).each_with_index do |order, index|
             - sum = order.sum
-            - total += sum
+            - transport = order.transport || 0
+            - total += sum + transport
             = ', ' if index > 0
             = link_to format_date(order.ends), new_finance_order_path(order_id: order)
-            = ' (' + number_to_currency(sum) + ')'
+            = ' (' + number_to_currency(sum)
+            - if transport != 0
+              = ' + ' + number_to_currency(transport)
+            = ')'
 
       %dt= heading_helper(Invoice, :number) + ':'
       %dd= @invoice.number