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.
should i not bother using an instance variable for this? is it slightly faster because it only queries the database once (whereas it's used twice in the view)?