Make student mastery outcome groups accessible

Fixes CNVS-14008

Test Plan:

1) Create a course and add a student
2) Turn on the Student Mastery Gradebook feature
3) Add at least 2 outcome groups each with at least 2 outcomes
4) Masquerade as the student, and visit the Learning Mastery grades tab
5) Verify with screenreader that outcome groups are clearly clickable

Change-Id: Ib9773d8275c7adb13783ee6b0acac551b59fc09b
Reviewed-on: https://gerrit.instructure.com/37702
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>