use aria-label instead of screenreader-only inside button
use aria-label instead of screenreader-only inside button

There is a known issue with JAWS and Firefox, where it does not ignore
aria-hidden elements inside of a button. Using aria-label on the button
works around the issue.

fixes OUT-2114

test plan:
* Using JAWS with Firefox in Windows, navigate to either the course or
  account outcomes page as a teacher and keyboard-navigate over the
  buttons in the toolbar. They should not read the button label twice.
* Confirm the behavior is identical with VoiceOver in Safari

Change-Id: Ic3565abd54e77e69f88de51227ddc5ed43d64661
Reviewed-on: https://gerrit.instructure.com/169256
Tested-by: Jenkins
Reviewed-by: Augusto Callejas <7d9b9fd126d23d111f693e51511b33ea0e7192f6@instructure.com>
Reviewed-by: Frank Murphy III <ed7a7068a7b39653c9ca1c429cffcb87c4a069c6@instructure.com>
QA-Review: Nathan Rogowski <2e8aa918660411855c6d44d5bb2da677aa033255@instructure.com>
Product-Review: Nathan Rogowski <2e8aa918660411855c6d44d5bb2da677aa033255@instructure.com>

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