use .build instead of .new when generating calendar event
use .build instead of .new when generating calendar event

It seems that for some courses, calling
course.calendar_events.new.context returns nil, instead of the course.
For other courses it works, I'm trying to determine what the difference
is, but using .build instead of .new fixes things to work correctly.

fixes #CNVS-2542

test plan: Since this only happens on some courses, it may be difficult
to reproduce until we know exactly what triggers the bad behavior. In
any case, you should be able to visit /calendar without seeing an error
page, and you should be able to create new calendar events from that
page.

Change-Id: I3d4c17676b39382f61ef8ebffb35cf47eec39fa5
Reviewed-on: https://gerrit.instructure.com/16526
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Clare Hetherington <40d6bb3ed7fef516c73274131d5a837c16de74fd@instructure.com>

HotChalk/canvas-lms,wrdsb/canvas-lms,HotChalk/canvas-lms,nditech/red-innovadores-lms,kyroskoh/canvas-lms,venturehive/canvas-lms,ShreniiNepal/shrenii_lmp,greyhwndz/canvas-lms,fronteerio/canvas-lms,fronteerio/canvas-lms,Unow/canvas-lms,arrivu/jigsaw-lms,arrivu/qibla-ecampus,ShreniiNepal/shrenii_lmp,SwinburneOnline/canvas-lms,djbender/canvas-lms,efrj/canvas-lms,Rvor/canvas-lms,roxolan/canvas-lms,jay3126/canvas-lms,tgroshon/canvas-lms,whalejasmine/canvas-lms,arrivu/hoodemo,nditech/red-innovadores-lms,redconfetti/canvas-lms,merichar/canvas-lms,kyroskoh/canvas-lms,HotChalk/canvas-lms,arrivu/jigsaw-lms,kyroskoh/canvas-lms,Jyaasa/canvas-lms,roxolan/canvas-lms,redconfetti/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,whalejasmine/canvas-lms,skmezanul/canvas-lms,va7map/canvas-lms,usmschuck/canvas,utkarshx/canvas-lms,juoni/canvas-lms,landonwilkins/canvas-lms,fronteerio/canvas-lms,loganrun/pincan,Jyaasa/canvas-lms,fronteerio/canvas-lms,usmschuck/canvas,loganrun/pincan,usmschuck/canvas,juoni/canvas-lms,venturehive/canvas-lms,Rvor/canvas-lms,HotChalk/canvas-lms,va7map/canvas-lms,samuels410/greatlakes-lms,leftplusrightllc/canvas-lms,redconfetti/canvas-lms,leftplusrightllc/canvas-lms,whalejasmine/canvas-lms,sli7246/canvas_branch,wimemx/Canvas,va7map/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,juoni/canvas-lms,SwinburneOnline/canvas-lms,parbros/red-innovadores-lms,loganrun/pincan,skmezanul/canvas-lms,efrj/canvas-lms,merichar/canvas-lms,jay3126/canvas-lms,shidao-fm/canvas-lms,arrivu/qibla-ecampus,sli7246/canvas_branch,wye0220/shindig-canvas,djbender/canvas-lms,Unow/ins,tgroshon/canvas-lms,Jyaasa/canvas-lms,wrdsb/canvas-lms,tgroshon/canvas-lms,wimemx/Canvas,redconfetti/canvas-lms,skmezanul/canvas-lms,utkarshx/canvas-lms,arrivu/hoodemo,parbros/red-innovadores-lms,greyhwndz/canvas-lms,djbender/canvas-lms,Unow/canvas-lms,parbros/red-innovadores-lms,leftplusrightllc/canvas-lms,merichar/canvas-lms,utkarshx/canvas-lms,Unow/ins,sfu/canvas-lms,efrj/canvas-lms,Unow/canvas-lms,antoniuslin/canvas-lms,grahamb/canvas-lms,arrivu/arrivu-lms,va7map/canvas-lms,sfu/canvas-lms,juoni/canvas-lms,kyroskoh/canvas-lms,nditech/red-innovadores-lms,ShreniiNepal/shrenii_lmp,sli7246/canvas_branch,rup/Tarrax,antoniuslin/canvas-lms,antoniuslin/canvas-lms,grahamb/canvas-lms,leftplusrightllc/canvas-lms,sfu/canvas-lms,nditech/red-innovadores-lms,usmschuck/canvas,samuels410/greatlakes-lms,Rvor/canvas-lms,sdeleon28/canvas-lms,ShreniiNepal/shrenii_lmp,Rvor/canvas-lms,roxolan/canvas-lms,landonwilkins/canvas-lms,landonwilkins/canvas-lms,efrj/canvas-lms,skmezanul/canvas-lms,sdeleon28/canvas-lms,SwinburneOnline/canvas-lms,shidao-fm/canvas-lms,shidao-fm/canvas-lms,greyhwndz/canvas-lms,sdeleon28/canvas-lms,sdeleon28/canvas-lms,rup/Tarrax,Unow/ins,samuels410/greatlakes-lms,roxolan/canvas-lms,wrdsb/canvas-lms,arrivu/arrivu-lms,wye0220/shindig-canvas,greyhwndz/canvas-lms,wimemx/Canvas,jay3126/canvas-lms,wye0220/shindig-canvas,arrivu/jigsaw-lms,sfu/canvas-lms,utkarshx/canvas-lms,wrdsb/canvas-lms,arrivu/hoodemo,venturehive/canvas-lms,landonwilkins/canvas-lms,Jyaasa/canvas-lms,arrivu/qibla-ecampus,whalejasmine/canvas-lms,jay3126/canvas-lms,grahamb/canvas-lms,shidao-fm/canvas-lms,tgroshon/canvas-lms,rup/Tarrax,arrivu/arrivu-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,antoniuslin/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,samuels410/greatlakes-lms