accessibility: fix a header label on student grades page
accessibility: fix a header label on student grades page

the 5th column, where the scoring details and comments are shown, did not have
a header label.  This adds one that is only visible to screen readers, since we
don't care to have one in the UI.

fixes CNVS-1335

test plan:
- go to a student's grade page
- with a screen reader, navigate the table header
- the 5th column should read "details"

Change-Id: Ic75c2488ae97884c0393817fa2abfe33f004390b
Reviewed-on: https://gerrit.instructure.com/17896
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Stanley Stuart <2b43fb8b7a234825d50dd49ce7892d78a59da8f3@instructure.com>
Reviewed-by: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>

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