Opdatering til #963: Brug exists() i stedet for count()

[rasmusselsmark]

Efter review fra @lakridserne på #963 (vis checkmark på invitation, om person allerede er tilmeldt aktivitet), ændret count() > 0 til exists()

[what-the-diff]

PR Summary

• Improvement to Participant Check in activityinvite_admin.py
  The method used to check if a person is participating in an activity has been improved. Before, we used to go through the entire list of activity participants to count them, but now we check for the existence of a person in the activity participants list, making the process quicker and more efficient.