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
If you are going to use an activerecord model in a migration, you should define the model inside of the migration, so `class Comment < ActiveRecord::Base; end`. this way, if you ever remove the model and try to run migrations, you won't run into problems with `Comment` not being defined