make feedback page always available; give students more information about feedback rules

[kjdav]

Kevin suggested making it so feedback pages are always clickable. If feedback is available, students see it; if feedback is not available, students should see a message saying feedback isn't available for this question.

It would also be good to provide students more information about feedback rules: timing, availability, when they have to view it, etc. It seems like we should put this information on the assignment page feedback settings. Unless the rules vary per exercise, which would be tricky.