Fixes for content import

This fixes #2642 and resolves #2641.

This also addresses concerns raised by @noemigerber about apparently misimported content.

Summary of changes:
* Properly construct parent_ids for nodes.
* Include organization info in topics.json to allow sidebar display.
* Fixes bug that prevented videos in the topic tree from having extraneous data removed.

@aronasorman Please take a look - once merged, it should be pulled to the demo server and content reimported.