fix scrollbar on ds assignment settings dialog
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 <ce3305c9be70e1a5a9ff49df19e1a5b24d5a47f8@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>
Product-Review: Matthew Irish <01acd3247a4780711d0930f64c5667e7367603a2@instructure.com>

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