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.
lml/ost#345: Fixes from @dantemss review.

- Remove the newly introduced `family` scope in favor
  of using association accessors.
- Remove extra closing tag (typo).
Add memoization for the `authority?` function.

- Add dependency for the `memoist` gem.
lml/ost#345: Handle JP's review comments.

- Use local variable in view/controller instead of helper.
- Separate exercise section rendering for instructor and
  student roles.
- Remove memoist gem.
- Remove url partial and inline using `exercise.url`
Merge pull request #349 from lakshmivyas/feature/linkqb

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