Stragglers display, exams cleanup

Another patch from another round of on-the-fly hacking.

* Quizzer now has a "Stragglers" button, to identify students
  who have neglected quizzes. At the online quiz stage of the
  workflow, this is more important information to us than
  right/wrong answer rates. Students have to make mistakes
  in order to improve.

* The monolitic exam generation code has been divided
  between admin/class (where we generate the exam paper
  and leave it open for final editing), and admin/quiz
  (where we provide a download button). Much better.