add missing 'Backbone' require
add missing 'Backbone' require

fixes CNVS-4627

test plan:
- with compiled assets enabled, go to a page that uses editor toggle (like quiz
  edit)
- it should not throw an error about missing Backbone

Change-Id: I3207157396b74daa85cf1b1d55f91135976610b3
Reviewed-on: https://gerrit.instructure.com/18578
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Stanley Stuart <2b43fb8b7a234825d50dd49ce7892d78a59da8f3@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>

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