models: fix touch not propagating when using nested attributes

Sometimes, when using nested attributes, touch doesn’t propagate to
parent relationships. (see https://github.com/rails/rails/issues/26726)

Specifically, this happens in our app when updating a dossier with
only new attachements (but without changing the value of any fields).

To work around this, we need to define the parent relationship
explicitely. This is good practice anyway.

Fix #3906

app/models/champ.rb

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