WL-168 When redeeming a Registration Code in the shopping cart, the user should be redirect to the Registration Code Redemption page

@chrisndodge @smagoun 

Kindly review the changes for WL-168

Thanks
Actually, I take this approval back. Dropping this column will break production during the time between running migrations and when the code gets deployed. @muhhshoaib @afzaledx can remove this migration and put the field back in the model (as a nullable field)? We'll have to clean this up at another point in time after we feel most people using Open edX has already updated their code.