[Outcomes] Add "mastery" column to student outcomes report

[CZaccagnino]

The current columns in the student outcomes report are "attempts" "latest score" and "percentage". These columns do not give us a sense of the students' mastery in the outcome, which is an important part of a student outcomes report. It would be extremely helpful to add a column for "mastery" that shows the students' level of mastery in that outcome. Then all the information for that student would be in one place. Thank you!