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
Avoid using activerecord in migrations - because what if the Comment model doesn't exist later, and someone runs the migration? To futureproof it , do this:

```ruby
# Null: TRUE! WOW
add_column :comments, :parent_id, :integer, null: true
add_column :comments, :parent_type, :integer, null: true

# Set parent_id column to the value of the post_id column, for each comment
update("UPDATE comments SET parent_id = post_id, parent_type = 'Post'")

# Disallow nulls again
change_column_null :comments, :parent_id, false
change_column_null :comments, :parent_type, false
```