Update an event on itip-attend page for logged-in users (Bifrost#T27847)

A participant can accept an invitation via link. This works fine
for initial invitation. However, if there is any update for the event
in another invitation request, the event in participant's calendar
was not updated.

Example scenario:

• an organizer invites a participant for specified date/time
• a participant declines the invitation (he's busy on that time), the event in participant's calendar will be created
• an organizer updates the event with another date/time
• a participant now accepts the modified event, the event should be updated with the new date/time.