Fix STUD-1316

This bug affected unpublishing content (and would trigger an error
message in studio). The fix was to uncouple the rendering of the studio_view
of an xblock with the student_view, so that content that wasn't in
draft mode already wouldn't be modified by early rendering of the
studio_view.

[STUD-1316]
Merge pull request #2628 from cpennington/fix-stud-1316

Fix STUD-1316