Add pagination with Kaminari

Previously: posts displayed as a huge list
Problem: with a realistic number of posts, this would get complicated for users and make it hard to keep your place when you look through a bunch of posts in a category
Solution: use the [kaminari gem](https://github.com/amatsuda/kaminari) to paginate posts. This required a new method, make_paginatable, to make ActiveRecord collections play well with kaminari.