notify screen reader users of posted assignemnt submission comments.
notify screen reader users of posted assignemnt submission comments.

fixes #9236

when a user posts an assignment submission comment, their screen
reader should now notify them of the new comment when it successfully
posts.

test plan:
  * using voiceover or another screen reader, navigate to the
    assignment submission page and post a comment;
  * verify that when the comment posts, it is read back by the screen
    reader.

Change-Id: Id2b2c304bc37173fc4b32866a61fb826fdc2d1a1
Reviewed-on: https://gerrit.instructure.com/13499
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Bryan Madsen <0a3ba1af216659500dd8f41e29f3979f42452870@instructure.com>

nditech/red-innovadores-lms,sfu/canvas-lms,greyhwndz/canvas-lms,ShreniiNepal/shrenii_lmp,gbleydon/canvas-lms,usmschuck/canvas,redconfetti/canvas-lms,fronteerio/canvas-lms,shidao-fm/canvas-lms,snehakachroo1/LSI_replica,arrivu/jigsaw-lms,wrdsb/canvas-lms,skmezanul/canvas-lms,tgroshon/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,skmezanul/canvas-lms,whalejasmine/canvas-lms,usmschuck/canvas,antoniuslin/canvas-lms,AndranikMarkosyan/lms,loganrun/pincan,whalejasmine/canvas-lms,grahamb/canvas-lms,arrivu/jigsaw-lms,djbender/canvas-lms,narigone/canvas,shidao-fm/canvas-lms,dgynn/canvas-lms,landonwilkins/canvas-lms,AndranikMarkosyan/lms,instructure/canvas-lms,arrivu/hoodemo,ankithbti/canvas_lms,matematikk-mooc/canvas-lms,ShreniiNepal/shrenii_lmp,rup/Tarrax,roxolan/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,utkarshx/canvas-lms,utkarshx/canvas-lms,HotChalk/canvas-lms,landonwilkins/canvas-lms,parbros/red-innovadores-lms,greyhwndz/canvas-lms,leftplusrightllc/canvas-lms,gbleydon/canvas-lms,leftplusrightllc/canvas-lms,instructure/canvas-lms,wimemx/Canvas,sfu/canvas-lms,samuels410/greatlakes-lms,Unow/ins,skmezanul/canvas-lms,arrivu/hiring-site,matematikk-mooc/canvas-lms,dgynn/canvas-lms,arrivu/arrivu-lms,loganrun/pincan,venturehive/canvas-lms,Jyaasa/canvas-lms,grahamb/canvas-lms,dgynn/canvas-lms,Rvor/canvas-lms,arrivu/qibla-ecampus,samuels410/greatlakes-lms,kyroskoh/canvas-lms,Unow/canvas-lms,loganrun/pincan,skmezanul/canvas-lms,kyroskoh/canvas-lms,antoniuslin/canvas-lms,sdeleon28/canvas-lms,jay3126/canvas-lms,grahamb/canvas-lms,ShreniiNepal/shrenii_lmp,eCNAP/CANVAS,snehakachroo1/LSI_replica,whalejasmine/canvas-lms,SwinburneOnline/canvas-lms,wye0220/shindig-canvas,merichar/canvas-lms,roxolan/canvas-lms,nditech/red-innovadores-lms,efrj/canvas-lms,sdeleon28/canvas-lms,eCNAP/CANVAS,arrivu/hiring-site,va7map/canvas-lms,Unow/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,ShreniiNepal/shrenii_lmp,utkarshx/canvas-lms,snehakachroo1/LSI_replica,va7map/canvas-lms,AndranikMarkosyan/lms,arrivu/qibla-ecampus,SwinburneOnline/canvas-lms,arrivu/arrivu-lms,landonwilkins/canvas-lms,wye0220/shindig-canvas,va7map/canvas-lms,leftplusrightllc/canvas-lms,matematikk-mooc/canvas-lms,parbros/red-innovadores-lms,juoni/canvas-lms,efrj/canvas-lms,arrivu/arrivu-lms,wrdsb/canvas-lms,fronteerio/canvas-lms,juoni/canvas-lms,sfu/canvas-lms,merichar/canvas-lms,AndranikMarkosyan/lms,efrj/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,Unow/ins,narigone/canvas,SwinburneOnline/canvas-lms,roxolan/canvas-lms,juoni/canvas-lms,va7map/canvas-lms,grahamb/canvas-lms,snehakachroo1/LSI_replica,tgroshon/canvas-lms,ankithbti/canvas_lms,wimemx/Canvas,sli7246/canvas_branch,sfu/canvas-lms,efrj/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,arrivu/hoodemo,Jyaasa/canvas-lms,shidao-fm/canvas-lms,shidao-fm/canvas-lms,instructure/canvas-lms,merichar/canvas-lms,tgroshon/canvas-lms,greyhwndz/canvas-lms,rup/Tarrax,Unow/canvas-lms,instructure/canvas-lms,antoniuslin/canvas-lms,usmschuck/canvas,utkarshx/canvas-lms,Jyaasa/canvas-lms,landonwilkins/canvas-lms,sdeleon28/canvas-lms,Jyaasa/canvas-lms,wimemx/Canvas,djbender/canvas-lms,redconfetti/canvas-lms,jay3126/canvas-lms,antoniuslin/canvas-lms,rup/Tarrax,instructure/canvas-lms,wye0220/shindig-canvas,usmschuck/canvas,narigone/canvas,instructure/canvas-lms,nditech/red-innovadores-lms,Rvor/canvas-lms,Rvor/canvas-lms,greyhwndz/canvas-lms,ankithbti/canvas_lms,leftplusrightllc/canvas-lms,whalejasmine/canvas-lms,eCNAP/CANVAS,narigone/canvas,SwinburneOnline/canvas-lms,arrivu/jigsaw-lms,wrdsb/canvas-lms,gbleydon/canvas-lms,fronteerio/canvas-lms,fronteerio/canvas-lms,arrivu/hiring-site,kyroskoh/canvas-lms,HotChalk/canvas-lms,Rvor/canvas-lms,jay3126/canvas-lms,redconfetti/canvas-lms,samuels410/greatlakes-lms,tgroshon/canvas-lms,nditech/red-innovadores-lms,dgynn/canvas-lms,parbros/red-innovadores-lms,Unow/ins,sli7246/canvas_branch,eCNAP/CANVAS,roxolan/canvas-lms,juoni/canvas-lms,arrivu/hoodemo,samuels410/greatlakes-lms,kyroskoh/canvas-lms,venturehive/canvas-lms,wrdsb/canvas-lms,redconfetti/canvas-lms,venturehive/canvas-lms,jay3126/canvas-lms,sdeleon28/canvas-lms,sli7246/canvas_branch,arrivu/qibla-ecampus