update attendee number in eventcards

[jangismarifet]

Task title
update attendee number in eventcards

Task description
In the events page: when you click LEAVE/JOIN, the button and the server updates. But the number of attendees in the event card does not.
Before clicking LEAVE:

After clicking LEAVE:

[github-actions]

Branch i153-update_attendee_number_in_eventcards created!