fix sorting buttons in student summaries table
fix sorting buttons in student summaries table

test plan:
 - in course analytics, the sortable headers should be clickable
   and clicking them should sort the table
 - enter/space should do the same

fixes ADMIN-2713

Change-Id: I8798f69d2a9ddfb0e9683a8b4f2c0d292a4066b8
Reviewed-on: https://gerrit.instructure.com/208404
Tested-by: Jenkins
Reviewed-by: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
QA-Review: Anju Reddy <851202533fb1f0b615e4547ba226c76996d060dd@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

instructure/analytics,instructure/analytics,instructure/analytics