champ: return the parent dossier even when discarded

Dossier has a `default_scope { kept }`.

Because of that, when the parent dossier is discarded, `champ.dossier`
will return nil.

We should kill the default scope. But meanwhile, ensure that
`champ.dossier` returns even a discarded dossier.

app/models/champ.rb

@@ -1,5 +1,5 @@
 class Champ < ApplicationRecord
-  belongs_to :dossier, inverse_of: :champs, touch: true
+  belongs_to :dossier, -> { with_discarded }, inverse_of: :champs, touch: true
   belongs_to :type_de_champ, inverse_of: :champ
   belongs_to :parent, class_name: 'Champ'
   has_many :commentaires