outcome gradebook: fix column resizing

fixes CNVS-10493

test plan:
  * enable and open outcome gradebook
  * resize a column label cell
  * verify that the aligned data cells resize gracefully

Change-Id: Ib6711179ca082cd0fb09891e91845b57c4890052
Reviewed-on: https://gerrit.instructure.com/28804
QA-Review: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>