Event price can change after registration start

One-sentence description

Event price can change after registration start

Current behaviour / Reproducing the bug

1. I register for an event accepting the costs etc
2. The costs change
3. I pay more than what I accepted
4. I'm angry 😠
5. Problems

Expected behaviour

Event prices cannot change after registration start!

---

This is also important for #632, since payments are made using the price in the event and if the price changes the payments cannot reflect that.