Specify 'default' functions for 'reference' field in SalesOrder and PurchaseOrder

- Refactor CreatePurchaseOrder form
- Some migrations

InvenTree/order/tests.py

@@ -60,12 +60,6 @@ class OrderTest(TestCase):
         order.save()
         self.assertFalse(order.is_overdue)
 
-    def test_increment(self):
-
-        next_ref = PurchaseOrder.getNextOrderNumber()
-
-        self.assertEqual(next_ref, '0008')
-
     def test_on_order(self):
         """ There should be 3 separate items on order for the M2x4 LPHS part """