Render detailed guides without a mainstream category

The view expects the detailed guide to have a primary mainstream
category when constructing breadcrumbs and producing a list of slugs for
CSS class names.
Rename filter method to set detailed guide breadcrumbs
Merge pull request #1319 from alphagov/render-detailed-guides-without-mainstream-categories

Render detailed guides without mainstream categories

https://www.pivotaltracker.com/story/show/67200884