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 making a helper so you can do `class: breadcrumb_link_class(link)`? Then you can avoid ternaries and use multi-line `if`s.