Added thread type attribute.

This PR is needed by the following [PR 5308] (https://github.com/edx/edx-platform/pull/5308) in order to function correctly.

@jimabramson @gwprice Please review.
In the models/comment_thread.rb, you define the callback like so:

```
after_update :foobar

def foobar
  if self.thread_type_changed?
    thread.comments.each do |comment|
      comment.update_attributes(:endorsed => false, :endorsement => nil)
    end
  end
end
```

I can't think of a great name for the function off the top of my head.