User can delete posts

Added a button in the posts edit view, allowing the current user to delete their own posts.
Relabeled "edit" links in the index and show pages to "edit or delete." 
I recommend doing away with the `before_action` (We typically try to avoid them whenever possible. They are harder to reason about because they are defined far away from where they actually happen, so sometimes you can miss that they are going to get called). I recommend defining a method on user `accessible_posts` that returns all posts for admins, and `posts` for anyone else. then just finding the posts through the current_user