CIText is displayed as a TextAreaField by flask-admin.
Override it to use StringField.

- A network defines an available range of IP addresses
- A host defines an IP address and name
- A host is always linked to a network
- A mac address is always linked to an item (physical equipment)
- A mac address can be linked to one host (IP)
- An item can have several mac addresses

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.

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