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
For i18n, please use string identifiers instead of `{0}`:
```
_("Oops! The code '{registration_code}' you entered is either invalid or expired".format(registration_code=course_reg.code))
```