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.
`@include_mathjax = true`'s only effect is to add in another JS include during the page load, so it isn't a security risk that needs checking.  If someone made it past the `SecurityTransgression` check above, they are going to see the page so might as well just say `@include_mathjax = true`.