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/locales/simple_form.en.yml

@@ -36,6 +36,7 @@ en:
         password: Password
         setting_auto_play_gif: Auto-play animated GIFs
         setting_boost_modal: Show confirmation dialog before boosting
+        setting_delete_modal: Show confirmation dialog before deleting a toot
         setting_default_privacy: Post privacy
         severity: Severity
         type: Import type