Fixed errors in discussion XModule

...f3725628b186fa:

1. Enabled `thread_type` selector for inline discussions
2. Fixed missing `commentable_id` (Javascript null was encoded as string 'null', not null object)

This addresses: https://openedx.atlassian.net/browse/TNL-606
@jmclaus due to [this](https://github.com/edx/edx-platform/blob/jmclaus/fix-errors-discussion-xmodule/common/static/coffee/src/discussion/views/discussion_thread_view.coffee#L281).