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
so this partial is used both in ```posts#show``` and ```coments#show``` but the main problem is that you need one comment (```comment```) to be the parent, because the POST polymorphic route thingy is [some_parent_comment, the_new_comment_whose_data_you_want_to_post] (```comments_comment_path```) 

so i need a form that holds data for the new comment, and i need a reference to the comment it is replying to. it's really confusing and i'm not sure how to get both without the instance variable