Add preference setting for delete toot modal (#3368)

* Set delete_modal preference to true by default
* Does not show confirmation modal if delete_modal is false
* Add ja translation for preference setting page

config/settings.yml

@@ -16,6 +16,7 @@ defaults: &defaults
   closed_registrations_message: ''
   boost_modal: false
   auto_play_gif: true
+  delete_modal: true
   notification_emails:
     follow: false
     reblog: false