Changed API (adapted for core visualization)

Changes:
- added description field in ``Rack`` model,
- added example fixture for accessories,
- added migration,
- change API to more django-rest way.
I'd rather go with something more explicit/descriptive/readable, e.g.:

```
current_height = int((assets.exclude(model__category__is_blade=True).aggregate(sum=Sum('model__height_of_device'))['sum'] or 0))
free_u = self.max_u_height - current_height
return free_u
```
(the first line should be wrapped of course :wink:)