Improved feedback options

Improve the feedback system by adding a couple of more sophisticated feedback options:

• A max score for a problem (!103 (closed) has made a start on this, but is rather old)
• A min score for a problem (Maybe zero can be a global default?)
• Feedback options that change the max score:
  • For example, "[] No units" would set the max score to 4, on an otherwise 6 point question or whatever
• Exclusive feedback options (basically radio buttons): pick one of these (#207 (closed))
• Add changing order of feedback options (#35)
• for lack of a better term: "Negative descriptions", descriptions of a feedback item that are sent to a student when a feedback option is not selected
• Super options: feedback options that include some other options (basically nesting), for example
  • Fully correct proof (6)
    • Base case correct (1)
    • Correct I.H. (2)
    • Correct inductive step (4) Selecting fully correct proof would then include the nested options.