74977456 'Articles model/controller/view specs refactoring'

https://www.pivotaltracker.com/story/show/74977456
(part of epic 74977456 https://www.pivotaltracker.com/epic/show/1288994)
In PR465, I did it with `allow(document).to receive(:friendly_id).and_return(document.title.parameterize)`. Is it better to assign `slug` instead? If we are going to use `build_stubbed` as much as possible, should we add `slug` to factories?