Sometimes it is necssary to display parts of a question outside of an
attempt. For example for staff review. When displaying the question, we
need to handle images in the question text. In the past there was a
mechanism for this that could only cope with the question text.

This commit introduces a new method that can handle displaying any
part of the question content.

This commit intentionally does not upgrade the parts of the system that
use the mecanism. That is so that this commit can be used to demonstrate
that backwards-compatibility works. The next commit will upgrade the
callers.