Add 'ticket_id' field to admin panel.
Fix bug with admin panel and 'wristband_id'.

Previously, the 'wristband_id' field was being submitted through an
admin ticket form. If unfilled, it would be blank, which caused
'params[:wristband_id]' to be equal to "". Since wristband_ids must be
unique, an admin adding two tickets would violate uniqueness.

This commit fixes the issue by checking if the 'wristband_id' field is
blank, both on ticket creation, and ticket update. If it is, the field
is instead set to nil.