use traitement model

when a dossier is terminated (accepte, refuse or classe_sans_suite),
we store now `processed_at` and `motivation` in a traitement instance

db/migrate/20200630140356_create_traitements.rb

@@ -0,0 +1,11 @@
+class CreateTraitements < ActiveRecord::Migration[5.2]
+  def change
+    create_table :traitements do |t|
+      t.references :dossier, foreign_key: true
+      t.references :instructeur, foreign_key: true
+      t.string :motivation
+      t.string :state
+      t.timestamp :processed_at
+    end
+  end
+end