Introduce Popular links

This PR is to introduce the Popular Links part of the Related Content work, which is a panel on the right hand side of articles.

Changes in this PR will probably want a quick review by frontend devs (cc: @benbarnett/@alexwllms ) as we've modified the main layout for all articles.

Work related to bringing in the popular links is behind a feature flag: 'popular_links'. The related for popular links can be found in the following PR https://github.com/moneyadviceservice/cms/pull/116
Try it. The Array() is smart enough to return an empty array that gets mapped over and subsequently does exactly what you guard for in the first case. 

Sent from my iPhone

> On 23 Dec 2014, at 16:27, Caden Lovelace <notifications@github.com> wrote:
> 
> In lib/core/entity/article.rb:
> 
> > @@ -20,5 +20,12 @@ def only_child?
> >      def callback_requestable?
> >        Repository::CallbackRequestable::Static.new(self).call
> >      end
> > +
> > +    def popular_links
> > +      return [] if related_content.blank?
> This line guards against related_content being blank — so I think the exception would still raise on related_content['popular_links'].
> 
> —
> Reply to this email directly or view it on GitHub.
>