make quiz answer titles render correctly for screenreader

fixes CNVS-14737

test plan:
  - after submitting a quiz, use screenreaders to navigate through the answers
  - note that each answer that has a colored flag next to it reads the flag
  - attempt to navigate to the answers given on matching, multiple answer, and
    other types of questions

Change-Id: I6cfe570a5b934a3bda4c592083b397dff9006af3
Reviewed-on: https://gerrit.instructure.com/41658
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>