manually backport #2275 app styles to stable

This is a manual backport of #2275, because it was a bunch of commits but essentially just a big addition.

It will not mess around with existing apps because it’s all new ID and class names. But it will ensure that the News and Notes app are compatible with 5.0.* without needing to duplicate these now (in master) standard app styles.

Please check @karlitschek @Raydiation @stefannagtegaal @butonic