Drag and drop on the course outline page

STUD-1939

@talbs here is the new branch
Do we really need to call this method here?
I thought it will be called in `render` method.
`this.refresh` -> [this.onXBlockChange](https://github.com/edx/edx-platform/blob/christina/drag-drop-outline-2/cms/static/js/views/xblock_outline.js#L182) -> [this.render](https://github.com/edx/edx-platform/blob/christina/drag-drop-outline-2/cms/static/js/views/xblock_outline.js#L185)