Payment: Add ability to unsplit Stripe Balance Transaction

[arborrow]

At the moment, there is no way to remove a Stripe Balance Transaction (SBT) that has more than one payment. In the case of refunds, it them makes it difficult to process the refund payout. While generally it is best not to give user's access to change a the SBT id associated with the payment, in this case it would be helpful. My thinking is that by updating the payment update method, we might check for the case where the amount is 0, the SBT id is not null and the payment method is reallocation. In that case, we could remove the SBT id and set it to null and append a note to the payment that the SBT id with the id number has been removed.