Finalize points for checkpoint challenge

[kasnerz]

We can now have both points for the checkpoint itself and points for the challenge at the checkpoint.

However, the participants have currently no way of claiming the points for the challenge.

For that, we need to:

• [x] add the checkbox for the challenge to the checkpoint form
• [ ] allow to do this sequentially (add the checkbox also to "Moje příspěvky"?)
• [x] make sure that the points for the challenge are counted on the Leaderboard