outcome gradebook: fill available window height

fixes CNVS-10629

test plan:
  * enable and open outcome gradebook
  * resize your browser window
  * verify that, as in assignment gradebook, all available
      vertical space is used by the result grid

Change-Id: Ic5455c4760f56331f3a2e776a2aeb22705212014
Reviewed-on: https://gerrit.instructure.com/30229
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>