Reschedule event notification even if previously sent

One-sentence description

Reschedule event notification even if previously sent

Current behaviour

Only the notification time is changed, even though it has been sent already.

Expected behaviour

If the notification was sent already it should be decoupled from the event and a new scheduled notification should be created.

Steps to reproduce

  1. Create event
  2. Let notification send
  3. Change the event start/registration times
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