Improve the overall feedback section of the quiz form

At the moment, when you create a new quiz, there are 5 overall feedback boxes, and when you edit an existing quiz, you get either (current number of feedback * 1.5) or 5 boxes, whichever is more.

(E.g. if the quiz had 5 feedbacks, then you get 7 boxes on the edit form. If it had 10, you get 15 boxes.)

This is not ideal. It takes up a lot of space on the form.

Our experience of the question type forms (e.g. answers and hints) is that once the quiz is created, the best thing is to have a number of form fields that matches the number of repeat items that actually exist, since adding more is a rare action, and so it is not a problem to click the "Space for 3 more ..." in that case. (We probably always need to show one box, to avoid errors.)

For new quizzes, we should pick a sensible number of boxes. The only data I have ready access to is the OU, and the data there shows:

From that, it looks like starting the form with 5 boxes would be right. (Before seeing the data, my guess was 2 empty boxes for a new quiz.)