Delegate Saveable `drawing_prompt` to question.

This missing delegation would explain how, as we saw in [#59636810], prompts could be published properly to the portal (as they apparently were) and yet not turn up in certain reports (which they weren't).

https://www.pivotaltracker.com/story/show/59636810
Strip tags from ImageQuestion prompts in report

These come in wrapped in `<p></p>` containers, which probably caused trouble.