Id vs slug regression

Somewhere along the way, we lost a bunch of khanload.py code.  I thought I resurrected it all; apparently not.

One important commit (dc1c4a01d020a822affe21aec77a4faa2335c4c7) was still missing, where we set id to slug.  If we don't do this, quite a few things break (because we mix using slug and id in different places, with the assumption that they're the same--they no longer are).

Easiest way to see this is that the student-specific coach report is broken.  Other things break, but can a bit harder to see.

Fix: reapply these changes, re-test.  Everything looks happy again!
Notes: https://github.com/learningequality/ka-lite/issues/233 tracks the broader issue