Display unique courses on the Grade page

when teaching 2 sections within the same course, we shouldn't display 2 links for one course.
Added a test to cover unique courses

fixes CNVS-6682

test plan:
1. Enroll a teacher to more than one section in a course
2. Click on Grades in the top nav
3. Make sure only one link is displayed for the course with multipled section enrollments
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Change-Id: I22ef5f437b405a8a33c48c182d51adf231ab75c4
Reviewed-on: https://gerrit.instructure.com/37318
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Brad Horrocks <bhorrocks@instructure.com>