Workaround for id != slug KA change

The KA "id" field completely changed between earlier and current topictree versions.  If we use their "id", any system that upgrades will fail to connect users to their exercise and video progress.

In addition, our code assumes in some places that "id" and "slug" can be used interchangeably.  The KA change breaks that assumption, and so those places in our code breaks.

Proposed change:
* When `khanload`ing, set id=slug (to reflect the previous state of affairs).  This works because slug is still unique.

Note:
* We need to get this in! current develop version disconnects users from their data.