Ex-74 Registration Code redemption

fix the translation issues

added a check if a user is already registered in a course. Changed the messages

added course depth=0 and removed pep8 violations
Ex-72-added additional billing information

Added a new CSV file in the instructor dashboard sales tab to download all the order sales separated from the invoice sales
Merge pull request #5259 from edx/muhhshoaib/Ex-72-WL-purchasers-provide-optional-information

Muhhshoaib/ex 72 wl purchasers provide optional information