- refactor Network class

- network name should be unique and not nullable
- force all name fields to StringField

Use full exception error as message (error messages from psycopg2 can
be quite long but that makes it much more explicit)

TODO: we should maybe raise ValidationError in models
and catch that to raise CSEntryError in the API

- Allow to pass several mandatory fields in create_generic_model
- Add tests

- catch IntegrityError on patch
- disable autoflush (IntegrityError was raised before commit due to
autoflush)

Pass ?qrcode=true as query string

Can use:

  -  /api/items?serial_number=123456
  -  /api/items?location_id=1

- 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

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