use more understandable 'link' icon for public links (instead of hard to recognize globe)

Fix #9707

> Currently the icon for a public link is a globe: ![](https://raw.githubusercontent.com/owncloud/core/master/core/img/actions/public.png) – although not really recognizable.
> The standard icon for a link though is a »chain link« icon. We should change the public icon (as well as the public folder icon) to reflect that.

Please review @owncloud/designers @karlitschek