Add a note about the avatars and banners situation

lib/pleroma/web/media_proxy/media_proxy_controller.ex

@@ -67,6 +67,9 @@ defp handle_preview(conn, url) do
     end
   end
 
+  # TODO: find a workaround so avatar_static and banner_static can work.
+  # Those only permit GIFs for animation, so we have to permit a way to
+  # allow those to get real static variants.
   defp handle_preview("image/gif" = _content_type, conn, url) do
     mediaproxy_url = url |> MediaProxy.url()