Makes conversation course filter accessible

fixes: CNVS-14514
CNVS-12805

test-plan:
1.) Make sure your user is in a group
2.) Navigate to conversations
3.) With screenreader on, navigate to course filter
4.) Verify that the drop down is navigable
5.) Navigate to compose message and verify that the
    course filter there is also navigable

Change-Id: I10d8f2131645adb59fab4a9b200591c1fd341010
Reviewed-on: https://gerrit.instructure.com/41915
Product-Review: Aaron Cannon <acannon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>