fix .screenreader-only in Chrome/Safari
fix .screenreader-only in Chrome/Safari

the class was incorrectly adding some extra space at the end of the page,
because of a screenreader-only class on the Instructure logo at the bottom of
the page.

fixes CNVS-9315

test plan:
- make sure text that is only availble to screenreaders is still not affecting
  the layout.
- make sure there is no extra space at the bottom of the page (in Gradebook2,
  for example, causing double scrollbars).
- make sure text in .screenreader-only classes is still read by screenreaders.
  for example, the logo at the bottom of the page should read as "Instructure,
  makers of the open source learning management system Canvas"

Change-Id: I2af474f3f7b04f932bf905d8e1c44d08e5d1fa79
Reviewed-on: https://gerrit.instructure.com/26047
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Aaron Cannon <25919c673621c006b735dcd5a28df63325948e2b@instructure.com>
QA-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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