feat(sales_order): Stock Items show the PO number

Stock items show the PO number that they were purchased on when being
viewed in the sales order allocation modal and when viewing the sales
order details.

InvenTree/stock/models.py

@@ -1370,6 +1370,12 @@ class StockItem(MPTTModel):
         if self.location:
             s += ' @ {loc}'.format(loc=self.location.name)
 
+        if self.purchase_order:
+            s += " ({pre}{po})".format(
+                pre=helpers.getSetting("PURCHASEORDER_REFERENCE_PREFIX"),
+                po=self.purchase_order,
+            )
+
         return s
 
     @transaction.atomic