fixed observers view of assignment index page

Fixed the observers view of assignments.  The
assignment index page will now show according
to whether an observer is observing a student,
multiple students, or the course itself.

fixes CNVS-16472

Test plan:
- Create a few users and a few observers and
  enroll them in the same course.
- Link one observer with one student and another
  observer with multiple students. Leave one
  observer with no linking.
- Create assignments, quizzes, discussions ect
  that are overdue, graded, submitted ect.
- Go to the assignments index page as each roll
  and check to make sure all assignments are
  categorized properly.

Change-Id: Ibe9000b11b91f80e6ac57672a38b1bffc5227455
Reviewed-on: https://gerrit.instructure.com/44721
Reviewed-by: Matt Berns <mberns@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Adam Stone <astone@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>