Renamed feature setting 'DISPLAY_HISTOGRAMS_TO_STAFF' to 'DISPLAY_DEBUG_INFO_TO_STAFF' to better communicate its purpose.

Default is True.

LMS-1199
Added feature setting 'DISPLAY_HISTOGRAMS_TO_STAFF' to configure display of histograms in Staff Debug Info panel.

Generating histograms requires scanning the courseware_studentmodule table on
each view. This can make staff access to courseware very slow on large courses.

Default is False.

LMS-1199
Merge pull request #2425 from edx/usman/lms1199-make-histograms-optional

Added feature setting to disable histograms in Staff Debug Info panel.