Special handling of credit card payments

[jantman]

All transactions are counted against the income amount for the pay period they're made in. This becomes problematic with transactions made on credit cards (as opposed to budget-funding accounts): if we make a purchase with a credit card and then pay it off during the same pay period, the amount is counted twice against our available income: once for the purchase and a second time for the payment.

Up until now, I've been working around this by manually calculating the sum of all purchases made with each card during the same pay period as the payment, and then entering a fake transaction negating that amount against the same account and budget as the payment.

Transactions should have an additional field to indicate that they're a payment towards a credit account; this field should be a dropdown that allows selecting the credit account that they're a payment towards. Then, when calculating the totals for each pay period, we'll subtract the sum of transactions on a given credit account from the sum of payments towards that account.