Wd 12322 modify credentials shop to use the new products and coupons

Done

Credentials now uses coupons to make the products freely available to everyone instead of a 0 price product

Check out this feature branch
Run the site using the command ./run serve or dotrun
View the site locally in your web browser at: http://0.0.0.0:8001/
- Be sure to test on mobile, tablet and desktop screen sizes
visit /credentials/shop
- Purchase an exam
- Exam should be priced at 0 and should be purchased successfully
visit /credentials/redeem
- Redeem enter an activation key
- See that the dropdown is populated with valid exam products
- Redeem the key
  - Check if exam is redeemed and visible in /credentials/your-exams

Fixes #WD-12322

[If relevant, please include a screenshot.]