Show new primary navigation for articles

Feature covers an article belonging in a single child category, within a
single parent category
Cover scenario of article existing in 2 categories within the same parent category
Cover scenario with an article residing in 2 categories within different parent categories
Make ArticleDecorator#categories public, we have other uses for it.
Use the decorated article
Create concern to handle setting active category
Use Navigation concern to specify currently active categories

The partial doesn't need to know about the current article now
Use CSS classes from styleguide