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

lib/tasks/deployment/20200630154829_add_traitements_from_dossiers.rake

@@ -0,0 +1,18 @@
+namespace :after_party do
+  desc 'Deployment task: add_traitements_from_dossiers'
+  task add_traitements_from_dossiers: :environment do
+    puts "Running deploy task 'add_traitements_from_dossiers'"
+
+    dossiers_termines = Dossier.state_termine
+    progress = ProgressReport.new(dossiers_termines.count)
+    dossiers_termines.find_each do |dossier|
+      dossier.traitements.create!(state: dossier.state, motivation: dossier.motivation, processed_at: dossier.processed_at)
+      progress.inc
+    end
+    progress.finish
+
+    # Update task as completed.  If you remove the line below, the task will
+    # run with every deploy (or every time you call after_party:run).
+    AfterParty::TaskRecord.create version: '20200630154829'
+  end
+end