Support rewards or incentives

[Steppschuh]

Add a reward section to the config to provide an incentive for users to provide feedback.
There should be a configurable probability that a user will get the reward after completing the survey.

Also add event tracking for rewards.

[Steppschuh]

The required backend functionality is implemented. The frontend still needs to:

• Check the config if rewards are enabled
• Get the reward probability
• Show a reward with the configured probability when the survey has completed
  • Get the QR code image from /api/reward/qr
  • Show the image and the reward message in a modal
  • Prevent the survey from resetting until the modal is closed

In addition, the /api/reward/ routes to get a rewards will need to be protected with an API key / secret (see FEEDBACK_KIOSK_SECRET). That secret could be provided to the frontend via an URL parameter or an input field on an extra page that saves it as cookie.