dossiers: save draft by default (instead of submitting)

Make the default behavior of `update_brouillon` be to update the draft,
instead of submitting the dossier.

This makes all requests made to `update_brouillon` without specifying
an extra `submit_draft` parameter to just save the draft. It will make
autosaving the draft easier and safer.

app/views/shared/dossiers/_edit.html.haml

@@ -43,13 +43,14 @@
         - if dossier.brouillon?
           = f.button 'Enregistrer le brouillon',
             formnovalidate: true,
-            name: :save_draft,
             value: true,
             class: 'button send secondary',
             data: { 'disable-with': "Envoi en cours…" }
 
           - if dossier.can_transition_to_en_construction?
             = f.button 'Déposer le dossier',
+              name: :submit_draft,
+              value: true,
               class: 'button send primary',
               disabled: !current_user.owns?(dossier),
               data: { 'disable-with': "Envoi en cours…" }