student learning mastery: afford clicks on groups

test plan:
  * open student learning mastery view
  * hover over an outcome group
  * verify that you want to click it

Change-Id: I3273167f7bc621b30a067184a849c6b5baf47af5
Reviewed-on: https://gerrit.instructure.com/36562
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>