hide content export <hr>s from screen readers
hide content export <hr>s from screen readers

fixes ADMIN-1286

test plan:
- Navigate to a course's Settings page
- Click on Export Course Content
- Move through the page with a screen reader
- Note that each horizontal rule is read to the SR

Change-Id: Ic04b9beba89f4cc9847528cb16189100123d79c8
Reviewed-on: https://gerrit.instructure.com/158913
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Luke Kingsley <379b4f41f09e1994f9aff7c4f09c9a2d6febd498@instructure.com>
Product-Review: Nathan Rogowski <2e8aa918660411855c6d44d5bb2da677aa033255@instructure.com>

djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms