add label to "Message Students Who..." select box

fixes CNVS-13733

test plan:
- go to individual gradebook view and turn on a screenreader
- select an assignment
- go down and click on the "Message Students Who" button
- it should read the first select box as "Message Type"

Change-Id: Ib3aa136457dd03f4da6dba934e6e05ced0ea0a81
Reviewed-on: https://gerrit.instructure.com/37682
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>