add screenreader notice for masquerading
add screenreader notice for masquerading

fixes CNVS-29026

Test Plan:
- In Safari with VoiceOver on, navigate to the Settings page of a course
- Click on the Student View button in the right column
- Navigate to first div in page letting you know you are masquerading
- Move to another page in the course
- Navigate to first div in page letting you know you are masquerading

Change-Id: If774dd2bdbd0bf736f465a9e8b50d1f2c3e16789
Reviewed-on: https://gerrit.instructure.com/78795
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
Tested-by: Jenkins
Product-Review: Aaron Cannon <25919c673621c006b735dcd5a28df63325948e2b@instructure.com>
QA-Review: Deepeeca Soundarrajan <5bcd4b623f00b7fc2942cd1c65c245b30a48a61f@instructure.com>

fronteerio/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,dgynn/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,grahamb/canvas-lms,roxolan/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,fronteerio/canvas-lms,fronteerio/canvas-lms,SwinburneOnline/canvas-lms,dgynn/canvas-lms,matematikk-mooc/canvas-lms,HotChalk/canvas-lms,dgynn/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,djbender/canvas-lms,matematikk-mooc/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,roxolan/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,SwinburneOnline/canvas-lms,roxolan/canvas-lms,instructure/canvas-lms