Add expiration dates to Coupon Codes

@chrisndodge @afzaledx Kindly review the changes for WL-135

Thanks
Although this is behind a feature flag, I know this won't get accepted by the Open edX folks, particularly since it hard codes a bunch of test data. I can give more feedback, but this might work better as a POST "api" call with the client passing along all of the parameters to set up the course_mode. (be sure to csrftoken protect it).

Then I think you can remove the feature flag.