a few things

Question breakdown header on the results screen changed to "Detailed question breakdown and feedback"
Text under that expanded.

The way a question counts as answered has changed - just answering one part is enough.
Fixed `question.answered` being set when question is revealed. I introduced `question.revealed` so I could keep the distinction, but forgot to stop `answered` being set!

Feedback icons in revealed questions are next to the student input, instead of the correct answer.

Don't show the part feedback box for parts with zero marks.