add screenreader text to grade by question popover
add screenreader text to grade by question popover

fixes CNVS-9090

test plan:
- on a grade by question quiz in speedgrader
- try to access the ? icon next to the fudge points box with a screenreader
- it should read the help text

Change-Id: I611ca0b8d31eae8e4d6d60b88c6beb5ac2e42bdf
Reviewed-on: https://gerrit.instructure.com/26217
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>
Reviewed-by: Cameron Sutter <853bcc0cb8f5834b7805e8eb9a762054be43c731@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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