Bugfix: TIM-588

[TIM-588](https://edx-wiki.atlassian.net/browse/TIM-588): Studio preview shows "You do not have permission to access staff information"

Turns out there were two underlying causes:
1) We were *always* sending an AJAX request for course staff info, even if there was nowhere in the template to put it (that is, even if we *knew* we weren't course staff).
2) Studio preview was including the section for course staff in the template, so the error message was getting displayed.

In this PR, I've addressed this issue by (1) skipping the AJAX call if there's no section in the container page for staff info (e.g. we know we're not course staff), (2) removing the course staff section in the container page when we're in Studio preview.

@gradyward Please review.