2018-2Q: Billing: Upgrade Subscription - Add to Next Invoice Date

A new option has been added to the "Upgrade Subscription" functionality in SIMPLer. Up until now, any package upgrades would result in an immediate Custom Invoice being generated and sent to the end customer. The new option "Add positions on next invoice run" allows the CSR to avoid sending an immediate invoice. Instead the increased fees will be added to the subscription table and will appear on the next routine invoice.