User sees the distribution of the current course grade of all learners the course

[crathbun428]

User Story
As a course author, I want to see a distribution of current course grades for all the learners in my course, so that I can view this information alongside other learner performance metrics for my course.

Acceptance Criteria
Scenario: A course author sees the distribution of current course grades for all learners in their course on the course dashboard
Given I am a course author with X permissions
When I view the learner performance section of the course dashboard
Then I see a distribution of current course grades for the learners in my course

[SoryRawyer]

I think this can be charted using the fact_problem_grades dataset in Superset.