Hides groups from courses that are unpublished

closes CNVS-17635
closes gh-97

Test Plan:
- Create a course that is unpublished
- Add a student to the class
- Add the student to a group
- Login as the student
- Go to the courses index page (/courses)
- The group should not be shown
- Publish the course
- The group should now be shown with a link to it

Change-Id: I3588cbd323cd065a33898c91021f475507101dee
Reviewed-on: https://gerrit.instructure.com/46266
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Clay Diffrient <cdiffrient@instructure.com>