adjust styles and markup for discount code field - 2669

See: https://github.com/easydigitaldownloads/Easy-Digital-Downloads/issues/2669

This is part of an effort to fix https://github.com/easydigitaldownloads/EDD-Site/issues/242 by adjusting EDD core (this PR) as well as Software Licensing https://github.com/pippinsplugins/EDD-Software-Licensing/issues/102