The hash is supposed to be computed in the __init__ method of Item.
But flask-admin never passes any parameter when creating the model class.
The attributes are changed after creation.

When creating an item in the admin view, we use the on_model_change
method to update the hash.

There might be a better way to avoid this code duplication.
This works for now.

- restrict admin view to admin users only
- the models shall only take optional parameters for flask-admin to work
- the models shall implement the __str__ method to be displayed properly
  in the admin view