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
It's the 'rails way'. Anywhere you then attempt to render singular or multiple ArticleLinks you get this for free. We should strive to use the established rails patterns for doing these things were possible. 

Sent from my iPhone

> On 23 Dec 2014, at 16:16, Caden Lovelace <notifications@github.com> wrote:
> 
> In lib/core/entity/article_link.rb:
> 
> > @@ -0,0 +1,11 @@
> > +module Core
> > +  class ArticleLink
> @benlovell Having looked into this, I'm not sure I can figure out how this saves us much. Since @article.popular_links is an array of ArticleLinks, the only way I can see of using this is to create a partial with this in:
> 
> <li class="related-links__link">
>   <%= link_to popular_link.title, popular_link.path %>
> </li>
> And pointing to_partial_path to that, which doesn't seem to save us much. Am I missing something?
> 
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