ValidationError is catched by default by flask-admin.
It's easy to catch in the api/main.py to raise a CSEntryError.

We actually don't need to catch ValidationError as it's based on
ValueError that we already catch.

Allow to use the same CSEntryError exceptions used for validation
in the API and admin views.

Trying to insert an invalid IP or network address raises an exception.
This is due to the unique validator from flask-admin that queries the
database.

The unique validator is automatically added for columns with
unique=True. The only way to disable it is to monkey patch the Unique
class.
This allows the field validator to catch invalid format.

IntegrityError are already catched anyway.

- replace TextAreaField by StringField for several Text columns
- add filter to replace empty string by None on optional Text and INET
  fields

Don't override the Admin view __init__ method for consistency.
Avoid having to import models in admin/views.py

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

The CIText type is not known by Flask-Admin and was not rendered.
A custom model converter is required.

See https://github.com/flask-admin/flask-admin/issues/1196

The serial number might not be unique and can't be used
as the string to hash.
There are for example SD cards that all have the same SN.

Using an ICS id allows to pre-print labels that can be assigned to
hardware when scanning serial numbers.
This id shall be easy to remember (unlike UUID).

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

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