Subscription items with 0 quantity

A new option has been added to SIMPLer settings that allows 0 quantity being used for subscriptions.

It can be enabled from Settings -> Modify WISP page under 'Billing - Zero quantity subscription' option.

It can be used in cases where an operator would like to show on the invoice that a particular product/service is available to a customer, but they have not purchased it for the moment.

The option is turned off by default.