Styles/Button: add .link style with image

[danirabbit]

Based on https://github.com/elementary/stylesheet/pull/923

There was a previous comment on the other PR about making this style an opt-in with an additional .external style class. Not sure if that's still desired or if this okay to trial for all links. Another option would be an opt-out .internal style class since the use case of a link button for navigating inside of an app is probably pretty obscure. I can't think of a case other than System Settings where a link would be used to navigate within the same app