A user can delete posts

Added logic to verify user who created post is the only one that can
see Delete link (didn't actually verify that logic worked)
Shout outs to Ifeanyi for holding my hand while I cried over instance variables

If we put `:destroy` with the shallow route for posts instead of the nested route, then the path can be shortened in `_post.html.erb`:

In routes.rb: `resources :posts, only: [:show, :destroy]`
In _post.html.erb: `<%= link_to "Delete Post", post, ...`

The controller would need to change slightly, too, since there won't be a `params[:location_id]`.