fix creating quiz groups from the banks dialog

fixes CNVS-10130

test plan:
  - Edit existing quiz
  - Go to the questions tab
  - "Find Questions" button
  - Select the group
  - In the "Add questions to the quiz group:" drop down, select "[New Group]"
  - Fill out the form with sane values
  - "Create Group" button
  - The group should be created successfully

Change-Id: I4197a3296d708b203ede265e7774456ec44ee894
Reviewed-on: https://gerrit.instructure.com/29763
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>