Add link in lms legacy instructor dashboard to cms course_index

A one-click way of getting to editing the course
Don't fetch the editor for XML courses
Address review comments for lms_link_to_cms

* Uses override_settings to provide test variable
* Move location of cms link to upper right
* PEP8 / Pylink
Merge pull request #859 from edx/jbau/studio-link-in-instructor-panel

Add link in lms legacy instructor dashboard to cms course_index