Run instructor-specified code as part of question setup

It's possible to imagine this for (multiple) text-response questions as well as (multiple) choice questions.

For choice questions, it's conceivable that
- the code generates options from scratch, or
- it just injects some stuff into the question's Jinja environment.

Ideally, this code would run in a container. Practically (i.e. for performance reasons), it probably won't--at least at first.