freeze student name column in outcome mastery gradebook

there were a couple of weird things to work around because we
custom implemented frozen columns in slickgrid by duplicating
the grid, and outcome mastery gradebook does some weird header
manipulations.

fixes CNVS-10625

test plan:
 - create a course with enough students and outcomes to require
   scrolling.
 - open outcome mastery gradebook and scroll horizontally and
   vertically. the header rows and the student name column
   should be frozen.
 - change the average calculation and the selected section and
   make sure there are no anomalies in the average header.

Change-Id: I383cf243291e9ab54dd744f0cf4188fe98846c97
Reviewed-on: https://gerrit.instructure.com/29001
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>