Re-enabling topic downloads from Khan Academy

Note that most of the work here was done by @rtibbles .  I'm just the messenger who did the merge / cleanup / small updates necessary to submit the pull request.

We are currently unable to update the topic tree due to a schema change in the KA API.  This code:
* Gets the new topic tree and spruces it up nice.
* Removes any new exercises (currently 14), as we aren't able to use these (yet)
* Updates all necessary json files (including topics.json, node_cache.json, maplayout_data.json, and others.

Note that we've chosen to strip out new exercises that we don't recognize (and not update existing exercises), as:
* We allow access to new videos immediately
* The number we strip out is very small (14 out of 474)
* Being able to update the topic tree is necessary to address #210