Group set drop-down is labeled

fixes CNVS-13939

Test plan:
* As a teacher, use NVDA and FF
* Go to Discussions
* When adding a new discussion, mark the checkbox to make it a group
  discussion
* Navigate to the combo box that appears which enumerates the various
  group sets available to pick from
* Ensure that the screen reader reads out that the combo box is for
  group sets

Change-Id: Ie0aa7e70251cb4bd396c52107ce542fb84cd0a8e
Reviewed-on: https://gerrit.instructure.com/45786
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>