conversations beta: fix layout for long threads
conversations beta: fix layout for long threads

test plan:
  * open conversations beta
  * open conversation with more than a screenful of messages
  * verify that you can scroll to the earliest

Change-Id: I0e011399aa5b9bedc266cbe759bbe4093ba2fec4
Reviewed-on: https://gerrit.instructure.com/23659
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>
QA-Review: Cam Theriault <b4133c36b6d893d0872cc09e87b536d16aa2d241@instructure.com>
Product-Review: Braden Anderson <a6cc2cef7d25f27847876bdb81651755fcebacf0@instructure.com>

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