add label to "Message Students Who..." select box
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 <ce3305c9be70e1a5a9ff49df19e1a5b24d5a47f8@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>
Product-Review: Liz Abinante <ce3305c9be70e1a5a9ff49df19e1a5b24d5a47f8@instructure.com>

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