Auto-Mark Invoices as Paid When Discounted to 100%

When an adjustment or discount brings an invoice's total to $0 (e.g. a 100% discount), automatically mark the invoice as paid rather than leaving it in sent/draft status waiting for a payment that will never come.

Useful for comped work, goodwill credits, or fully-discounted invoices issued purely as a record, there's nothing to collect, so the invoice shouldn't sit in an unpaid state indefinitely.