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
definitely not. But i'm not sure how to set sensible defaults. I guess
```comments = comments.all```
```comments.each do |comment|```
```  comment.parent_id = comment.post_id```
```  comment.parent_type = "post" ```
end

and then add the null constraint after