Get rid of course_index shim.

@dmitchell Please review. Let's cleanup as we go along instead of creating shims.

Things to test:
1) Create a new course
2) Import into existing course, click to go to course overview.
3) Export of something that fails (I wish I knew how to do this...). Should take you to the course overview page.
4) From unit page, click on Section in RHS to go to the course overview page.