add discount status translation

Based on this issue: https://github.com/easydigitaldownloads/Easy-Digital-Downloads/issues/2104

I can't say this is the best way to add the translation for the discount status. I couldn't seem to translate the string at any other stage, though.

Check my comments here (https://github.com/easydigitaldownloads/Easy-Digital-Downloads/issues/2104) and let me know what you guys think.