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
Please use
```javascript
commentableId = this.isTabMode() ? this.topicView.getCurrentTopicId() : void(0),
postData = {
  title: title,
  thread_type: threadType,
  body: body,
  commentable_id: commentableId
}
```
in this case we do not need lines
```javascript
if (this.isTabMode()){
  postData.commentable_id = commentableId;
}
```
values that have `undefined` value  $.ajax does not send.