Add functionality to custom invoice sent out dates
Currently, you can send out an invoice based on their term. Example:
Send out an invoice 7 days before due date for monthly
Send out an invoice 30 days before due date for yearly
This works *great*, however if you have free domains, you can't use the feature at all. Therefore WHMCS should be able to send out invoices with custom dates, except when there's a free domain associated with the order. If there is a free domain, it should use the standard domain invoice "sent out date" so they get combined.