add accessibility to Group Set combobox \o/

fixes CNVS-13120

test plan:
- using NVDA with Firefox
- go to the assignments#edit page
- verify that the "Group Assignment" combobox is accessible
- verify that you can use the whole combobox
- verify that you navigate to "Peer Reviews" combobox next

Change-Id: Ibc996d7831793f04cd314acf463c462cbe58d541
Reviewed-on: https://gerrit.instructure.com/38880
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>