Allow Payments by Payment Term in document Electronic Payments dialog

[HKuz]

Currently, when a user is logged into their desk view and makes an electronic payment directly from an order/invoice, the only option is to pay the entire total, and the payment schedule isn't updated under the hood. This feature would display the payment terms for the given document in the dialog, and enable them to select which one(s) to pay.

Implementation notes:

• All function calls look for a "payment_term" key in the data argument, which holds the name of the payment schedule's payment term to pay off. (This feeds into the payment amount and fee calcs throughout, and is used to update the payment schedule)
• Currently, all functions, including create_payment_entry and create_journal_entry, only expect one payment term - we'll need to refactor some code to be able to handle multiple terms