Add forums, topics and posts; add breadcrumbs; style css

1. Add forums which have topics and posts 
2. Add pagination with kaminari
3. Add breadcrumbs with gretel
4. The files in stylesheets/refills I didn't write, so in the interest of your time you might not care to review that
How about `[posts.count - 1, 0].max`?

I avoid having variables with the same name as the method (or any other method, ideally) because it can make it very confusing to read.