use a slave for Course#tabs_available
use a slave for Course#tabs_available

test plan:
 * visit several different courses

Change-Id: I19e8fbefba6164392269d6688df2c58d25e0f34b
Reviewed-on: https://gerrit.instructure.com/13027
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
Tested-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

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