Add previous and next article functionality.

__This commit branches off of #116, so [this diff](https://github.com/moneyadviceservice/cms/compare/popular_links...previous_next_article) is probably easier to follow.__

See moneyadviceservice/frontend#857.

Exposes the next and previous posts in the article's first category through the article JSON response.
You can use the @results as a subject rather than use the instance variable here.