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.
We can divide this in two contexts e.g:

```ruby
context 'when previous link' do
  subject(:previous_link) do
    subject.related_content[:previous_link]
  end
  
  before do
    allow(article).to receive(:previous_page).and_return(previous_article)
  end
  
  context 'when has previous article' do
    let(:previous_article) do
      build(:page, site: site, label: 'Previous Article', slug: 'previous-article')
    end
    
    it 'returns article title' do
      expect(previous_link[:title]).to eq('Previous Article')
    end
    
    it 'returns article path' do
      expect(actual_previous_link[:path]).to eq('/en/articles/previous-article')
    end
  end
  
  context 'when does not have previous article' do
    let(:previous_article) { nil }
    
    it 'is empty if no previous article' do
      expect(previous_link).to be_empty
    end
  end
end
```