Braintree Payments deposit / partial payment integration

Allow users by Role to pay an initial deposit instead of the total order amount through the Braintree payment gateway. This is useful for various scenarios, e.g., prepayment for a special order item, deposit for pickup, provision of a multiple partial payment plan, etc. A minimum amount can be specified and enforced. Orders that have outstanding amounts are listed as awaiting payment. Depending on settings, one or more further payments can be made through the Orders Awaiting Payment page.