Perseus integration, updates from KA API, and sidebar topic navigation

This is not ready to merge.

Wanted to put this up here so that @aronasorman can at least start to take a peek at this code.

This pulls all of Nalanda branch (not including latest updates in rct2 as yet) into develop, and has the integration of Perseus and the use of the playlist slidebar for topic tree navigation.

It also pulls in the latest Khan Academy topic tree.

Things that need to be fixed before merging:

* Perseus and Khan Exercises playing nice together
* Home page display
* Knowledge Map
* Content download for Perseus - images and assessment items json data