Add the invoice number to the billing status of entries with a status of 'Billing Complete"

I think it would be super helpful to add the invoice number to the billing status of entries that are 'billing complete". This way it becomes super easy to hunt down the invoice that entry is attached to. The only way to do this now is to pull a backup.

For example you could use this text, "Billing Status: Billing Complete (Invoice #10101)