Update aria-hidden to "true"
Update aria-hidden to "true"

we used aria-hidden="hidden" in a couple places
which should be aria-hidden="true"

test plan:
Navigate to Discussion and Annoucement and make sure the
icon in the subscribe button is not read to VO

Change-Id: I321f02b63c2e6413cbcf2de670290a8d66500586
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/214084
Tested-by: Jenkins
Reviewed-by: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
QA-Review: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
Product-Review: Daniel Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>

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