Outstanding balance uses reimbursement

[chrishummel]

This updates outstanding_balance to use the value of "reimbursements" when determining the payment total.

Currently outstanding_balance (when a refund exists) is defined as

total - (payment_total + refunds.sum(:amount))

Because refunds are included in the payment_total here the outstanding balance ends up equalling the "order total minus your completed payment total, regardless of whether those payments have been refunded or not".

In a couple scenarios this does not work as expected:

• User places an order with one payment method
• They reach out and request to switch payment methods
• Admin refunds payment and submits a new payment
  In this case the payment total will be double the order total and the order will show as refund owed.

In any scenario where the order is modified after payment is complete:

• User accidentally orders Product A (price $100) instead of Product B (price $10)
• Before Product A is shipped, user requests a switch to Product B
• Admin removes Product A, adds Product B, and refunds $90