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 
> Can we be sure that @showView is always be present here?

```rerender``` is only called from instances of DiscussionThreadView, so I do not see why it wouldn't be the case:
https://github.com/edx/edx-platform/pull/5554/files#diff-8c0fe1a4d82aa10a447f30150fd61dd5R110

But it's quite easy to add a ```if @showView?``` otherwise. @jimabramson What do you think?