provide status code on promotion handler

This change adds a status_code to the JSON response for the promotion code response object. It's a little bit brittle to bind on the translation message from ```Spree.t``` and sometimes more granularity is needed then success/error. The status_code should hopefully solve this.
cherry pick change pending on Spree, update checklist
Merge branch '2-3-dev' of github.com:blueapron/spree into 2-3-dev