Proof of concept of embedding Prooftoys proofs in an exercise

Add "rough proof of concept" exercise expression_expansion.html with
the Prooftoys proof builder embedded.

There is also a tiny fix to khan-exercise.js.

In particular, the Prooftoys proof editor is essentially unmodified, so among other things:

 * The step displays are cluttered, especially for simple exercises like this.
 * Implementation of the integration is basically just a demo.

This is my first pull request with you all and I am also a git novice, so please don't assume I have a clue on your procedures. Thanks to Marcos for help in the chat room on Monday.

I am quite excited to be doing this!  Thanks.