Include Attendees collected data in Reservation complete emails sent to admin

I could be cool (useful) if those "Reservation complete" emails sent to event admins included not only participants names, categories, Reservation id, Ticket identifier etc. but additionally the Attendees' data to collect (from the `/attendee-data-to-collect`).

We use it (Attendees' data to collect) for "Avez-vous déjà participé à un Scratch Day auparavant?" and "D'où avez-vous eu connaissence de ScratchDay?" and it would be neat to see the replies to that as registrations come in. (We can, of course, go into `/admin/` to see the _Statistics_, so this is just a, minor, enhancement suggestion.)

@cbellone @syjer