CTA gradient fix and tracking

The main change here is actually [_mixins.scss](https://github.com/guardian/frontend/blob/open-cta-show-more-fix/common/app/assets/stylesheets/_mixins.scss#L215).

The other changes are because someone has changed the JSHINT and not actually made the JS pass so couldn't commit (I don't like commitment issues).

![Dang it](http://stream1.gifsoup.com/view/768279/ronaldo-doh-o.gif)