Integrate Stripe CTF level 3.

This is an early pull request to collaborate on the required steps per level:

1. ~~Make it run on Cloud Foundry (covers the technical aspects of the hosting environment)~~
1. Make it functional and secure on Cloud Foundry (covers the aspect of being exploitable in the desired way, and not via other means; obviously this doesn't need to be handled too strict given the restricted use case, i.e. any other exploit discovered by participants might actually yield a new level ;)
1. Make it customizable per user (covers injecting the secret per level, but not the provisioning of a level ensemble)