add padding for tinyMCE images
add padding for tinyMCE images

test plan:
  - insert an image using tinyMCE. It should have some default padding
    around the top,right,bottom, and left of the image.

fixes CNVS-5956

Change-Id: I0256198e1674eef2897acbda44417a846f3bc659
Reviewed-on: https://gerrit.instructure.com/20849
Reviewed-by: Ryan Florence <2d8900489f91ac272fd9ce9d5789db6522668ca2@instructure.com>
Product-Review: Ryan Florence <2d8900489f91ac272fd9ce9d5789db6522668ca2@instructure.com>
QA-Review: Ryan Florence <2d8900489f91ac272fd9ce9d5789db6522668ca2@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>

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