User sees learner group performance data for each assignment

[crathbun428]

User Story
As a course author, I want to view learner performance details for each assignment in my course for learners in my learner group, so that I know how many of my potentially at-risk learners have attempted the assignment to date and can see how they are are performing on the assignment on average.

Acceptance Criteria
Scenario: A course author can see the number of learners in my learner group who have attempted each assignment and how those learners are have performed on the assignment on average
Given I am a course author with X permissions
When I view the performance section of the learner group reports
Then I see the number of learners in my learner group who have attempted at least one problem in each assignment
And the average breakdown of correct versus incorrect responses to the assignment across all learners in my learner group who attempted the assignment in the course