Add link in lms legacy instructor dashboard to cms course_index

A one-click way of getting to editing the course from the LMS instructor panel.

@sarina do you mind doing giving me comments on PR (or point me towards others)?  The test coverage, etc is all there.  

Basically, we've had faculty request an easier way of getting to edit course content from the instructor panel in LMS.  I think there's more specific links for editing (in github) if the content is xml-backed, but linking to the CMS course_index would be enough to satisfy our faculty.

I realize it uses the legacy dashboard, but maybe that makes the merge considerations less stringent? =)