Braintree Payments deposit integration

Allow users by Role to pay a deposit instead of the whole amount when they submit orders through the Braintree payment gateway. This is useful for various scenarios, e.g., prepayment for a special order item, deposit for pickup, providing a multiple partial payment plan, etc. The minimum amount to be paid can be specified and enforced. Orders that are not fully paid are listed as awaiting payment. Depending on settings, one or more further payments can be made through the Orders Awaiting Payment page.