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
I'd suggest to use:
```javascript
postData.courseware_title = this.topicView.getFullTopicName();
```
As I can see you start using `postData`. In this case, you should use `postData` instead `newAttrs` and remove `newAttrs`, because it's redundant (contains the same data as `postData`).