[IMP] website_event: avoid reading of emails

The new route make_event_ics_file received a list of attendee ids and was sending
the emails associated with these in an ics file, this could potentially
give out some information about the attendees.

(see commit 7862b2b7)

Also the method get_ics_file had no reason to be public and so is now
private.

Closes #24711