Re-add Promotion.with_coupon_code
Re-add Promotion.with_coupon_code

The implementation is quite simple and still makes sense with the new
muilti-code implementation, we might as well deprecate this before
removing it.

lsirivong/solidus,Arpsara/solidus,devilcoders/solidus,scottcrawford03/solidus,richardnuno/solidus,Arpsara/solidus,ckk-scratch/solidus,scottcrawford03/solidus,ckk-scratch/solidus,bonobos/solidus,devilcoders/solidus,athal7/solidus,athal7/solidus,pervino/solidus,grzlus/solidus,jordan-brough/solidus,Senjai/solidus,Arpsara/solidus,richardnuno/solidus,grzlus/solidus,bonobos/solidus,bonobos/solidus,pervino/solidus,scottcrawford03/solidus,forkata/solidus,forkata/solidus,lsirivong/solidus,bonobos/solidus,scottcrawford03/solidus,xuewenfei/solidus,devilcoders/solidus,jsurdilla/solidus,Senjai/solidus,ckk-scratch/solidus,Arpsara/solidus,richardnuno/solidus,grzlus/solidus,forkata/solidus,jordan-brough/solidus,athal7/solidus,lsirivong/solidus,Senjai/solidus,pervino/solidus,jsurdilla/solidus,grzlus/solidus,athal7/solidus,forkata/solidus,ckk-scratch/solidus,jordan-brough/solidus,xuewenfei/solidus,Senjai/solidus,richardnuno/solidus,xuewenfei/solidus,jsurdilla/solidus,xuewenfei/solidus,jordan-brough/solidus,jsurdilla/solidus,pervino/solidus,lsirivong/solidus,devilcoders/solidus