Let users reply to comments

Previously: comments only replied to posts
Problem: reddit has really neat threaded conversations of comments replying to comments replying to comments, replying to (etc)
Solution: 
-make comments belong to a polymorphic "parent" which they are responding to (either posts or other comments)
-make the _comment partial recursive, so that a comment displays all its replies just below it in a nested list
does that mean that if ```parent = Post.find(params[:id])``` that ```redirect_to parent.to_partial_path``` will redirect the user to _post.html.erb? because that's not the behavior I want for posts -- i'm using the _post partial for ```post#index``` and it doesn't display a post's comments--only the link to the show page