Add ablity to serialize an existing quantity of stock

- Do not have to serialize all the stock
- Add tests
- Add function to copy entire stock transaction history

InvenTree/part/fixtures/part.yaml

@@ -43,6 +43,7 @@
     name: 'Widget'
     description: 'A watchamacallit'
     category: 7
+    trackable: true
 
 - model: part.part
   pk: 50

InvenTree/part/models.py

@@ -843,7 +843,8 @@ class Part(models.Model):
         # Copy the BOM data
         if kwargs.get('bom', False):
             for item in other.bom_items.all():
-                # Point the item to THIS part
+                # Point the item to THIS part.
+                # Set the pk to None so a new entry is created.
                 item.part = self
                 item.pk = None
                 item.save()