Convert all actex directives to questions?

[aerenchyma]

We'll need to build questions themselves in order to have them searchable in the question bank interface; right now we do not. (Basically, put them all in the formatting described in this issue: RunestoneInteractive/RunestoneComponents#206 for summative exercises, i.e. all of the problem sets and presumably also the pre-lecture challenge questions.)

This is a semi-large QA/editing task, with formatting.