fix: AA-646: Fix is_staff_user check to check for course staff
fix: AA-646: Fix is_staff_user check to check for course staff

It was checking for global staff, but really only needed to check
for course staff. This corrects the check.

arbrandes/edx-platform,angelapper/edx-platform,arbrandes/edx-platform,eduNEXT/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,edx/edx-platform,edx/edx-platform,edx/edx-platform,angelapper/edx-platform,arbrandes/edx-platform,edx/edx-platform