Add user preference to always expand toots marked with content warnings (#8762)

config/settings.yml

@@ -27,6 +27,7 @@ defaults: &defaults
   delete_modal: true
   auto_play_gif: false
   display_sensitive_media: false
+  expand_spoilers: false
   preview_sensitive_media: false
   reduce_motion: false
   system_font_ui: false