fix scrollbar on ds assignment settings dialog

fixes CNVS-10983

test plan
=========
- is a browser that shows scrollbars and with draft state on, go to the
  assignments index page
- ensure that you have a few assignment groups
- click the settings cog in the top right
- verify that there is no additional scrollbar if your assignment groups
  don't scroll off the end
- resize the dialog via the bottom right corner and verify that when the
  dialog is too short, the assignment groups part of the dialog scrolls

Change-Id: Ic16675cd0928cc0aa42b69591c605171a2ccaf34
Reviewed-on: https://gerrit.instructure.com/30045
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Matthew Irish <mirish@instructure.com>