New tab (Metrics) in instructor dashboard

Metrics tab shows student data:
  -Count of students opened a subsection
  -Grade distribution per problem

for each section/subsection of the course.

Implemented for both the old and beta dashboard
Controlled by a feature flag 'CLASS_DASHBOARD'
Data is aggregated across all students
Aggregate data computed from courseware_studentmodule
Merge pull request #2346 from edx/dcadams/student_metrics_tab

New tab (Metrics) in instructor dashboard