add hover/focus states for star icons to hint at action

As mentioned by @tanghus in https://github.com/owncloud/core/commit/25e9b7a7423da5d8631d365250e7e5e5955b87b2#commitcomment-5563957

Please review @owncloud/designers 

This one should be backported as well, just like #7472 – ok @karlitschek?