Events on personal ical feed are now excluded if someone deregistered

Closes #963 (closed)

Previous behaviour

Steps to reproduce:

  1. Deregister from event
  2. Download personal iCal feed
  3. See the event despite you deregistered

New behaviour

Steps to validate that it works:

  1. Deregister from event
  2. Download personal iCal feed
  3. No event there

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