Specify order currency (#4698)

* Add 'order_currency' to the various external order models

- By default will use the currency specified for the supplier (or customer)
- Can be specified per order, also

* Display order currency on order pgae

* Add 'order_currency' field

* Enable "blank" currency option (to default to the currency specified by the referenced company

* Fix default currency code when adding line items

* Remove 'total_price_currency' serializer field

- Now replaced with 'order_currency' for greater flexibility

* Bump api_version.py

* Update default order report templates

* Updated docs

* More docs updaes

* Adjust unit tests

* Use 'order_currency' in order tables

* Update purchase order api unit tests

InvenTree/report/templates/report/inventree_po_report_base.html

@@ -70,7 +70,7 @@
             <td></td>
             <td></td>
             <th>{% trans "Total" %}</th>
-            <td>{% render_currency order.total_price decimal_places=2 currency=order.supplier.currency %}</td>
+            <td>{% render_currency order.total_price decimal_places=2 currency=order.currency %}</td>
             <td></td>
         </tr>
 

InvenTree/report/templates/report/inventree_so_report_base.html

@@ -70,7 +70,7 @@
             <td></td>
             <td></td>
             <th>{% trans "Total" %}</th>
-            <td>{% render_currency order.total_price currency=order.customer.currency %}</td>
+            <td>{% render_currency order.total_price currency=order.currency %}</td>
             <td></td>
         </tr>
     </tbody>