Most items won't have a name.
hostname could be added in a separate table if required.

User and Group shall not be created locally.
They all come from the LDAP/AD server.

Allow to easily pass config for testing

The citext module provides a case-insensitive character string type.
Use CIText for QRCodeMixin name column.
name should be case-insensitive to avoid duplicate names.

- We don't care about the id of the models. No need to force it.
- Rename actions

Use the tablename instead instead of an abbreviation.
The scanner client can thus directly get the name of the fields to use.

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