Add ghost logo anchor tag

Amends #3736, references #3623

With `button.ghost-logo`, there's no `href`, so cannot be opened in a new window. This changes it back to an anchor and appends the blog URL to a href attribute. Win!

Bumps Ghost-UI version to 0.8.13 bring in related CSS changes.
Merge pull request #3740 from PaulAdamDavis/ghost-logo-link

Change Ghost logo link back to an anchor