Updated topics.py to correctly create the nodecache from latest KA API.

Updated nodecache and topics.py.

Updated Video navigation for new keys.
Bug fix: Khan Academy had removed related videos from exercises data.

Now retrieved via separate API call.
Updated topics.py to correctly download all topic data.

Also now generate knowledgemap data, and download icons.

Missing icons are replaced by the default icon.
Script to pull latest Khan-Exercises repository.

KA Lite platform is not currently compatible with latest exercises.
Remove knowledgemap utils - this functionality is now within topics.py