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.
An initial step to prepare the creation of coupon module. The main goal
is to just move the logic from sale_coupon to coupon. To come up with
less diff as possible, this rename/move step is beneficial for this
purpose.
Joseph Caburnay