Sales & Rental: add custom fields

The client has a custom field for order number. Because the projects are short-term they do not want to use projects for this. This field is active for receipts, but they would like it to be active for sales & rental as well.

Make extra fields module active for sales & rental. Then show these extra fields in backend/app. Later migrate this also for the custom fields at receipt. Take into account that these custom fields can be transferred to invoice lines.