add User#accounts current-shard-only association
add User#accounts current-shard-only association

refs CNVS-8958

and fix User#all_accounts to just use it in the implementation

Change-Id: I0164f906ef083e2c38f744f63addd7d6217d8966
test-plan: existing specs
Reviewed-on: https://gerrit.instructure.com/25877
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Product-Review: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
QA-Review: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>

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