Store credit, applied automatically at checkout

Issue credit to a customer organization and it's applied by default on their next order.

Credits from promotions, goodwill, and returns are easy to forget. Claddio now tracks store credit for each customer organization, with the amount, remaining balance, expiry, and the order it came from.

At checkout, available credit is applied by default. When a return is processed, staff can issue account credit instead of a refund.