Add an explicit setting to determine whether a promotion requires a coupon code to apply

Describe your bug or feature request.

Currently, a promotion that doesn't reference any coupon is applicable right away.

This is confusing for merchants, as it isn't really obvious that saving a promotion without any condition will result in a promotion applied to every single order.

I'm suggesting we add a checkbox (Label: "Require coupon code", description: "When checked, applying a coupon code is required for the promotion to apply").

The checkbox should be unchecked by default (for backward compatibilty reasons).