[WIP] Multiple coupon per order

Fixes issue #1499.
@stloyd Then we will need to do same for `Sylius\Component\Core\Model\Taxonomy`, `Sylius\Component\Core\Model\Product`, `Sylius\Component\Core\Model\ProductVariant`...which is out of the scope of this PR.

Feel free to submit RFC about this if you think it's worth it.