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
@polesye
> it is redundant line of code, because we already set that attribute in line 83 this.model.set(postData).

Indeed, it looks like redundant code and if I remove it, everything works fine. @jimabramson Is there an obscure reason why you added this line that we do not know of? Thanks.