Add option to skip some items on a subscription

[chrean]

There are some use cases where, on a subscription, users may need to skip the delivery of some items. This may happen e.g., when a subscription contains items consumed with different frequencies, causing some to stockpile.

Adding an option to skip some items from the next shipment would solve this problem.

For additional context, one possible way to solve this at the moment is by updating the actionable_date on the SubscriptionLineItem model. Then, when processing the subscription, one would only charge the user for the items whose actionable date is equal to or before the current date.