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 
> I think we can use postData instead newAttrs. Just update it with courseware_title in tab mode.

Got rid of ```newAttrs``` and just use ```postData``` that is extended with ```courseware_title``` in tab mode,