Add link in lms legacy instructor dashboard to cms course_index

A one-click way of getting to editing the course
fix simple things because, damn
Don't fetch the editor for XML courses
Merge pull request #863 from edx/ned/fix-simple-things-because,-damn

fix simple things because, damn
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
Fix selector.
Fix test.
Merge pull request #868 from edx/hotfix/lose_header_bug

Fix bug when editing video causes other components to lose their header