Implement totalQuantity to Order type

[villesau]

Is your feature request related to a problem? Please describe.

When showing cart, it's nice to be able to show the total quantity next to cart. This is only possible by iterating trough the order lines. However, would be nice if I could query total quantity of products from the backend easily as a single number.

Describe the solution you'd like

Implement new field to Order type that returns total quantity of products in order:

query order {
  activeOrder {
      total
      totalQuantity <--- this
  }
}