POS: ad-hoc payments to a member's balance

You can now make ad-hoc payments to a member's balance from the POS app.

Here's how it works:

1. Just click on the same link that shows any unpaid/open invoices from the POS. If there are no open/unpaid invoices for that member, you'll see a link that reads "+ pre-pay balance"
2.  Enter the amount the member wants to pay, and click Next
3. Card reader will pull up the charge amount, and process the payment
4. The payment will show up in the member's account immediately, and we'll add a credit to their balance. If the balance becomes sufficient to close out any open invoices, then our system will close them out automatically.