Enable khanload to download latest topic files.

Fix for #1795, with the following caveats:
* Knowledge map construction is broken, and needs to essentially be implemented from scratch (#1796)
* I am not updating the topic tree now, because:
    * #1796 - knowledge map won't update
    * The # of exercises we support reduced from 435 to 415.
    * Need to check how many videos are not part of our torrent files.

So, bottom line: this fix has no impact, except to update the `khanload` command for further testing.