mail_template_sale_coupon is specific to sale_coupon so we are moving
it back to sale_coupon module. Additionally, we add a way of selecting
a mail template when generating coupons for customers.
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