Show a summary of what's new in the app #16805

Could be similar to cloud (the third icon):

when clicking on it

Ideally plugins can contribute such notifications as well.

We wouldn't mention every feature here but the most important ones. We could also regularly mention new FAQ's, new Guides etc.

To reuse existing code see Cloud/templates/whatsnew.twig and the Menu.php.

Should we also show this to anonymous when view access? I'd say lets have it disabled for now.