Ex-74 Registration Code redemption

@chrisndodge we have added the functionality related to EX-74, Kindly review the changes and let us know if anything is missing 

Thanks
One additional comment, it'll probably help with translation efforts if this whole above message is a single string with .format(course_name=course.display_name, site_name=sitename). Since it is broken up into separate strings right now, each line would be a different translation string and it might not make sense for translators.