Change from thin to unicorn

More processes per Dyno:
* http://blog.codeship.io/2012/05/06/Unicorn-on-Heroku.html
* http://stackoverflow.com/questions/9792290/on-heroku-cedar-should-i-use-thin-or-unicorn
* http://kuon.goyman.com/2012/heroku_unicorn/
Remove number_of_articles_without_discussions
Don't suppress navbar on welcome page