`Dossier.champs` is not really an inverse of `Champs.dossier`: when a
Champ record is created, it should not always be added to dossier.champs
(for instance if the champ is private).
NB: this breaks the workaround we added in #3907 to fix the parent
dossier not being touched in some cases (the workaround was to add an
inverse relationship, but we now have to remove it).
The new workaround is to watch for `changed_for_autosave?` on champs.
Unlike `changed?`, `changed_for_autosave?` also detects changes to
attachments. This allows us to touch both `last_champ_updated_at` and
`updated_at` in a single pass.
3499f5af9a