* extract parent for yes no and checkbox champs
* checkbox stores true / false instead of on / off
* normalize blank value to nil
* normalize invalid value to false
* after party task: normalize checkbox values
* after party task: normalize yes_no values
- Make `champ.dossier` a requirement;
- Move the dossier_id assignation to `before_validation` (otherwise
the record is invalid, and never gets saved);
- Allow specs to only build the champ (instead of saving it to the
database), which bypasses the requirement to have a dossier.
