Fixed grade_histrogram() so that it returns results for display in Staff Debug Info.

grade_histogram() was returning no results if any of the StudentModule rows had grade == Null.
However, grade is Null if the student has only viewed a problem and not attempted it. This
meant that histograms were almost never being displayed (since the probability that everyone
who has viewed the problem has also attempted it is effectively 0).

LMS-2209
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