Don't ovveride SQLAlchemy __init__ method on db.Model classes.

See http://flask-sqlalchemy.pocoo.org/2.3/quickstart/:
SQLAlchemy adds an implicit constructor to all model classes which
accepts keyword arguments for all its columns and relationships. If you
decide to override the constructor for any reason, make sure to keep
accepting **kwargs and call the super constructor with those **kwargs to
preserve this behavior

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.

The database constraints will catch the same error
(and raise an IntegrityError).

Having some validation at the model level allows to display nicer error
messages.
We use CSEntryError exception and not ValidationError so that it can be
used both by the API and views.

- 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

Text is as efficient in Postgres and is recommended over varchar.
It avoids having to put an arbitrary max length to strings.

CSE makes it clearer that it's linked to CSEntry

ics_id, serial_number, manufacturer_id and model_id should not change.
Repeating them in the history table takes space for nothing.

We are only interested in tracking the status, location and parent.

Pass ?qrcode=true as query string

- add more tests
- patching an item update children location and status
- setting the parent on an item updates the location and status with the
  parent ones

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).

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

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.

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