allow sorting of outcome gradebook columns

fixes CNVS-10635

test plan:
  * navigate to outcome gradebook;
  * verify that header cell popup now displays on hover
    instead of click;
  * click an outcome column header and verify that the row
    sorts ascending; click again and verify that it sorts
    descending. the header will have a white background
    while sorted;
  * click on the student names header (reads "Learning
    Outcome") and verify that student names can be sorted
    ascending and descending as well. note: students will
    be sorted by their sortable_name, which is usually
    "lastname, firstname."

Change-Id: I2f148d31f4c5576e86188730c2976bb711a203b7
Reviewed-on: https://gerrit.instructure.com/28902
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>