fix logo javascript for IE. fixes #11231
fix logo javascript for IE. fixes #11231

was previously querying 'background' css prop. on logo, which
IE was returning as 'undefined.' now queries 'background-image'
which works in all browsers.

test plan:
  * in IE8, log into canvas and load any page;
  * verify that no javascript error is thrown and that page
    functions as expected.

Change-Id: I78492bf42229042aa99a2723c90d83521b95cc0d
Reviewed-on: https://gerrit.instructure.com/14365
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>

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