lml/ost#345: Add questions with links to exercises section.

- Add `family` scope and method to the assignment model.
- Add `_questions` partial to show the exercise questions.
- Add `show_url` param and ability to the `_question` partial.
- Add `authority?` view helper function.
- Use `authority?` view helper function.
- Fix copyrights.

TODO:

- The styles for questions are a little weird.  Leaving things
  as is to wait for the UX review.
An instructor can sign up to be a student in a class (I'm pretty sure).  The way the new question list is nested inside `if @student_assignment.blank?` means that such an instructor wouldn't get to see the new list.  So maybe pull it out?