CTA gradient fix and tracking

The main change here is actually [_mixins.scss](https://github.com/guardian/frontend/blob/aed14f1811871ae02194b495a3f56f4eb29ef543/common/app/assets/stylesheets/_mixins.scss).

The other changes are because someone has changed the JSHINT and not actually made the JS pass.

![Dang it](http://stream1.gifsoup.com/view/768279/ronaldo-doh-o.gif)