hide Nav tray <hr> from screen readers
hide Nav tray <hr> from screen readers

fixes ADMIN-1287
fixes ADMIN-1290

test plan:
- Go to a course's Settings-Navigation tab
- Click on a kebab next to a nav item and choose "move"
- Move through the tray with a screen reader
- Horizontal rule should not be read

Change-Id: Ia34f3653ba2e1103fd2a3929f0479dc43d590c7e
Reviewed-on: https://gerrit.instructure.com/158951
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Luke Kingsley <379b4f41f09e1994f9aff7c4f09c9a2d6febd498@instructure.com>
Product-Review: Nathan Rogowski <2e8aa918660411855c6d44d5bb2da677aa033255@instructure.com>

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