models: require belong_to associations on type_de_champ

- Make `type_de_champ.procedure` a requirement;
- Move the procedure_id assignation to `before_validation` (otherwise
the record is invalid, and never gets saved);

spec/factories/type_de_champ.rb

@@ -7,6 +7,8 @@ FactoryBot.define do
     mandatory { false }
     add_attribute(:private) { false }
 
+    association :procedure
+
     factory :type_de_champ_text do
       type_champ { TypeDeChamp.type_champs.fetch(:text) }
     end