scaffold and seeds for work orders

This commit it much more than a generated scaffold:

1. Work order scaffold is super simple, it has couple of added
   fake fields, but its only purpose is to group 4 orders at a
   time.
2. Orders now belong to a work order, and have a box attribute.
   This is maybe not the cleanest way forward, but is find for
   the demo and it is fully incapsulated, and can be changed later.
3. Order items now have a priority field. The idea is to sort them
   by a product id, this way same products are conjoined and similar
   are near - because we seeded products by category from an actual
   shop.
4. Work order has_many orders, but also has many items, with a custom
   getter method.
5. Seeds do exactly what is explained above: they create one work
   order for each 4 consecutive rders and then 'randomize' the items
   priority (which, mind you, is not random at all but very much
   the opposite, and simulates meaningful pickup order).
6. Finally, work order allows viewing of orders as well as pick up
   order of items.

Signed-off-by: Zeljko <zeljko@zwr.fi>