allow peer_reviews_assign_at to be mass-assigned
allow peer_reviews_assign_at to be mass-assigned

refs #4118

Change-Id: Ie0c6f3f507be8a25dcfe9b5561578124c7ff1303
Reviewed-on: https://gerrit.instructure.com/3549
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@instructure.com>
Reviewed-by: Brian Whitmer <760e7dab2836853c63805033e514668301fa9c47@instructure.com>

eCNAP/CANVAS,loganrun/pincan,narigone/canvas,sdeleon28/canvas-lms,ottenhoff/canvas-lms,wimemx/Canvas,parbros/red-innovadores-lms,djbender/canvas-lms,greyhwndz/canvas-lms,arrivu/hiring-site,fronteerio/canvas-lms,ankithbti/canvas_lms,juoni/canvas-lms,arrivu/hiring-site,shidao-fm/canvas-lms,landonwilkins/canvas-lms,arrivu/jigsaw-lms,eCNAP/CANVAS,arrivu/jigsaw-lms,shidao-fm/canvas-lms,efrj/canvas-lms,usmschuck/canvas,juoni/canvas-lms,grahamb/canvas-lms,em2/canvas-lms,juoni/canvas-lms,efrj/canvas-lms,samuels410/greatlakes-lms,Jyaasa/canvas-lms,dgynn/canvas-lms,ShreniiNepal/shrenii_lmp,sfu/canvas-lms,skmezanul/canvas-lms,matematikk-mooc/canvas-lms,jay3126/canvas-lms,wrdsb/canvas-lms,narigone/canvas,rup/Tarrax,nditech/red-innovadores-lms,va7map/canvas-lms,kyroskoh/canvas-lms,snehakachroo1/LSI_replica,merichar/canvas-lms,SwinburneOnline/canvas-lms,dgynn/canvas-lms,dgynn/canvas-lms,ottenhoff/canvas-lms,grahamb/canvas-lms,gbleydon/canvas-lms,sli7246/canvas_branch,rup/Tarrax,wimemx/Canvas,va7map/canvas-lms,leftplusrightllc/canvas-lms,grahamb/canvas-lms,matematikk-mooc/canvas-lms,jay3126/canvas-lms,HotChalk/canvas-lms,leftplusrightllc/canvas-lms,arrivu/hiring-site,instructure/canvas-lms,parbros/red-innovadores-lms,sdeleon28/canvas-lms,matematikk-mooc/canvas-lms,kyroskoh/canvas-lms,AndranikMarkosyan/lms,Unow/ins,antoniuslin/canvas-lms,ShreniiNepal/shrenii_lmp,loganrun/pincan,arrivu/arrivu-lms,landonwilkins/canvas-lms,em2/canvas-lms,instructure/canvas-lms,kyroskoh/canvas-lms,arrivu/jigsaw-lms,wrdsb/canvas-lms,ottenhoff/canvas-lms,utkarshx/canvas-lms,nditech/red-innovadores-lms,Jyaasa/canvas-lms,faraazkhan/canvas,arrivu/arrivu-lms,ShreniiNepal/shrenii_lmp,faraazkhan/canvas,ShreniiNepal/shrenii_lmp,Unow/canvas-lms,Rvor/canvas-lms,sdeleon28/canvas-lms,Unow/ins,narigone/canvas,whalejasmine/canvas-lms,va7map/canvas-lms,AndranikMarkosyan/lms,AndranikMarkosyan/lms,fronteerio/canvas-lms,landonwilkins/canvas-lms,sli7246/canvas_branch,leftplusrightllc/canvas-lms,instructure/canvas-lms,Unow/canvas-lms,rup/Tarrax,skmezanul/canvas-lms,tgroshon/canvas-lms,tgroshon/canvas-lms,grahamb/canvas-lms,snehakachroo1/LSI_replica,venturehive/canvas-lms,antoniuslin/canvas-lms,parbros/red-innovadores-lms,utkarshx/canvas-lms,icelab/canvas,ankithbti/canvas_lms,wrdsb/canvas-lms,antoniuslin/canvas-lms,merichar/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,eCNAP/CANVAS,sfu/canvas-lms,whalejasmine/canvas-lms,roxolan/canvas-lms,arrivu/hoodemo,nditech/red-innovadores-lms,faraazkhan/canvas,Unow/canvas-lms,wimemx/Canvas,usmschuck/canvas,sfu/canvas-lms,samuels410/greatlakes-lms,sli7246/canvas_branch,efrj/canvas-lms,snehakachroo1/LSI_replica,arrivu/qibla-ecampus,redconfetti/canvas-lms,samuels410/greatlakes-lms,va7map/canvas-lms,Rvor/canvas-lms,icelab/canvas,greyhwndz/canvas-lms,instructure/canvas-lms,whalejasmine/canvas-lms,usmschuck/canvas,wye0220/shindig-canvas,greyhwndz/canvas-lms,instructure/canvas-lms,shidao-fm/canvas-lms,redconfetti/canvas-lms,redconfetti/canvas-lms,venturehive/canvas-lms,whalejasmine/canvas-lms,merichar/canvas-lms,sdeleon28/canvas-lms,greyhwndz/canvas-lms,AndranikMarkosyan/lms,shidao-fm/canvas-lms,arrivu/qibla-ecampus,sfu/canvas-lms,gbleydon/canvas-lms,djbender/canvas-lms,loganrun/pincan,sfu/canvas-lms,wrdsb/canvas-lms,tgroshon/canvas-lms,utkarshx/canvas-lms,redconfetti/canvas-lms,leftplusrightllc/canvas-lms,wye0220/shindig-canvas,instructure/canvas-lms,roxolan/canvas-lms,narigone/canvas,sfu/canvas-lms,eCNAP/CANVAS,snehakachroo1/LSI_replica,skmezanul/canvas-lms,venturehive/canvas-lms,arrivu/hoodemo,SwinburneOnline/canvas-lms,djbender/canvas-lms,icelab/canvas,HotChalk/canvas-lms,Jyaasa/canvas-lms,dgynn/canvas-lms,kyroskoh/canvas-lms,fronteerio/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms,SwinburneOnline/canvas-lms,skmezanul/canvas-lms,arrivu/arrivu-lms,fronteerio/canvas-lms,Unow/ins,gbleydon/canvas-lms,efrj/canvas-lms,utkarshx/canvas-lms,sfu/canvas-lms,jay3126/canvas-lms,nditech/red-innovadores-lms,arrivu/hoodemo,Rvor/canvas-lms,ankithbti/canvas_lms,roxolan/canvas-lms,samuels410/greatlakes-lms,jay3126/canvas-lms,HotChalk/canvas-lms,wye0220/shindig-canvas,arrivu/qibla-ecampus,Jyaasa/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,antoniuslin/canvas-lms,Rvor/canvas-lms,grahamb/canvas-lms,usmschuck/canvas,em2/canvas-lms,landonwilkins/canvas-lms,juoni/canvas-lms,roxolan/canvas-lms,tgroshon/canvas-lms