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.
I like this approach to add the file here rather than Comfortable mexican sofa :+1: :+1: :+1: 

But this class is a model, so I think we can add this file in app/models/comfy/cms/page.rb with the following:

```ruby
require ComfortableMexicanSofa::Engine.root.join('app', 'models', 'comfy', 'cms', 'page.rb')

class Comfy::Cms::Page < ActiveRecord::Base
  # ...
end
```

Wdyt?