* store prefill params in session
Instead of using query params on /dossier/new, we assume the user comes
from /commencer/:path, which is the new prefill link.
There, we store the prefill params in session, and use them to prefill
the dossier when creating it, in /dossiers/new.
* spec: cover the case
* review: serialize with json instead of yaml
* review: rename method
* review: store only query params
* review: comment why we dont override already stored params
