This module introduces compatibility of product packs with e-commerce:

- In the cart summary, the components aren't editable and are shown hanging
  from the main pack reference.
- When we remove a pack from the cart, their components are removed as well.
- The cart popup summary only shows the main pack line and discards the sublines in
  the units count.
- The cart summary shows the component lines hanging from the main one as well.
- It's ensured the the prices are shown correctly for the whole pack and that they're
  correctly summarized depending on the pack type.
