Grading endpoint for interactive and in-video exercises -- master

- 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.  Cleaner this way.  If we ever want to do per-problem grading of formative exercises, it can use this as well.
- 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.
    - log-formatted data written to ExamRecordFieldLog
    - net scores written to ExamRecord
- There are a bunch of configuration bits needed for interactive exercises.  Moved most of the settings from the HTML (where they had been "data-*" attributes, into the XML section, where they are better structured tags.  This is both more reliable and secure.  Also more flexible.  Fixes #1620.