Unify feedback endpoint; store log & net score; interactive autograder

- Moves the logic for communicating with the db-class interactive grader
  into the autograder.  Fixes #1569.
- Allows both interactive exercises and invideo quizzes to now use one
  feedback endpoint
- Feedback endpoint no longer uses non-standard query parameter.
- Correctly store log of interactive attempts and net scores for both
  interactive exercise and in-video quizzes.  Fixes #1449.
- Moved most of the settings to drive interactive exercises from HTML
  into the XML section.  This is both more reliable and secure.  Also
  more flexible.  Fixes #1620.