Change Ghost logo link back to an anchor

Amends #3736, references #3623

With `button.ghost-logo`, there's no `href`, so cannot be opened in a new window. This adds an anchor and appends the blog URL to a href attribute. :doughnut: 

Related styles: https://github.com/TryGhost/Ghost-UI/commit/e6c24979d1672e2e31e93aaed5e0d74a778db229