(Headline) Correct footer layout bug for sites with long copyrights

[cathysarisky]

This PR fixes the Headline theme footer, where the copyright element does not wrap. It's a problem particularly on mobile for sites with long titles or long custom text in the @custom.footer_text variable. nowrap is being inherited from gh-foot, so we need to explicitly set wrap for the copyright element.

[markstos]

I also searched all the other themes to see if they also mention .gh-copyright in screen.css, but only Solo matched and it's CSS wasn't quite the same.

[JohnONolan]

The underlying cause is white-space: nowrap on .gh-foot - which seems incorrect. Would be better to remove than override, if that's the case. @minimaluminium do you know if there's a reason for it?