Fulfillment: Keys for checkboxes should be order ID, not zero-indexed

[joeholdcroft]

Currently in fulfillment the keys for the checkboxes (and also the orders) are just zero-indexed. I think there was a reason for this, something to do with Symfony form.

However this is dangerous because it assumed the keys for the orders array and the checkboxes (same for dispatch checkbox listings like post/pick-up) have the same indexes.

This recently caused an issue when we started filtering out orders and the keys now had gaps.

The fix would be to always have the keys as the order ID so there was never a chance of the wrong order being changed.