Make it possible to pass query params instead of idlist to attendee mail

Make it possible to pass query params instead of idlist to attendee mail

When doing for example signup emails to large signups the list of ids
can be very long, and the materialization also gets delayed (so the
response status may have changed since the email was initiated). For
this reason, allow passing down query parameters other than just integer
ids, and expand those when creating the email rather than ahead of time.