Add swap.refunded final swap state for normal swaps

[kilrau]

Currently backend is unaware of refunds of normal swaps. It'd be nice to globally know which swaps were refunded by adding a swap.refunded final state for refundable normal swaps, meaning after invoice.failedToPay and also swap.expired. That would let web app query the state for features like below instead of saving this info locally which is error prone.