Fix TIcket reassign check

[marcoacierno]

I believe we changed the Pretix tickets/attendee-tickets endpoint to pick all positions where the position email and order email match the user (I believe it was done to make it work with hotels, social events etc?) which broke this check:

https://github.com/pythonitalia/pycon/blob/12b48d2e0862874b2850f6522c94eeceaf0132cb/backend/api/pretix/mutations.py#L61

The check assumes that when a ticket is re-asigned it is not returned by get_user_tickets: https://github.com/pythonitalia/pycon/blob/5c65e063bce37336ee2845c4da2db10f2e395513/backend/api/pretix/query.py#L38 but it is actually is. Maybe we should rename the function to make it more clear of this behaviour too?