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.

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