Removes get_assessment_items() khan command. Unauthorized endpoint.
Adds "all_assessment_items" lazy related field to Exercises.
Shows more verbose output when no JSON object decoded.
Prevents non-video nodes being included in get_videos returned list.
Prevents errors on processing assessment_items.
Stops validation from complaining about perseus items missing html.
Adds 'id' to denormed attribute list.
Merge commit 'dc3f06635da0389a18b1ce39a37742c10e6c8ea8' into hackathon
Updates videos.json to include full set of Khan Academy videos.
Merge branch 'hackathon' of github.com:learningequality/ka-lite into hackathon