This change restructures coupon functionality so shared coupon logic lives in a more generic coupon module. This makes it easier for different sales-related apps, such as helpdesk sales coupons, to reuse and extend coupon features consistently without duplicating work.

This refactoring extracts parts of sale_coupon module to create a more
generic module called coupon. This new module can be extended in
different sales modules to implement functionalities specific to modules
that extend it.