Exclude user_id from item versioning

The user_id is used to track the item creator. It's not supposed to be
updated afterwards. There is no need to track changes.

app/models.py

@@ -282,7 +282,7 @@ class CreatedMixin:
 
 class Item(CreatedMixin, db.Model):
     __versioned__ = {
-        'exclude': ['created_at', 'ics_id', 'serial_number',
+        'exclude': ['created_at', 'user_id', 'ics_id', 'serial_number',
                     'manufacturer_id', 'model_id']
     }