upgrade `brandable_css` so it uses less memory
upgrade `brandable_css` so it uses less memory

the way this was written, it used to fire off
a promise for every brand all at the same time.

I’ve changed it so the memory usage will be more
like `k * <# variants>` instead of
`<# of themes> * <# variants> * k`
so it will stay constant (on my machine ~500mb) and
not grow huge as we get more people that have made
themes.  (where k is the memory it takes to
compile one sass file, and # themes is a constant,
in our case 6.

Change-Id: I1fa4b4410a1141b461019f0c614ba343ea73000c
Reviewed-on: https://gerrit.instructure.com/59182
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

juoni/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,skmezanul/canvas-lms,instructure/canvas-lms,leftplusrightllc/canvas-lms,redconfetti/canvas-lms,greyhwndz/canvas-lms,narigone/canvas,tgroshon/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,whalejasmine/canvas-lms,leftplusrightllc/canvas-lms,efrj/canvas-lms,antoniuslin/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,fronteerio/canvas-lms,instructure/canvas-lms,greyhwndz/canvas-lms,skmezanul/canvas-lms,utkarshx/canvas-lms,roxolan/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,efrj/canvas-lms,dgynn/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,venturehive/canvas-lms,kyroskoh/canvas-lms,juoni/canvas-lms,HotChalk/canvas-lms,utkarshx/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,matematikk-mooc/canvas-lms,tgroshon/canvas-lms,leftplusrightllc/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,whalejasmine/canvas-lms,redconfetti/canvas-lms,instructure/canvas-lms,sdeleon28/canvas-lms,snehakachroo1/LSI_replica,jay3126/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,whalejasmine/canvas-lms,HotChalk/canvas-lms,dgynn/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,kyroskoh/canvas-lms,whalejasmine/canvas-lms,redconfetti/canvas-lms,snehakachroo1/LSI_replica,kyroskoh/canvas-lms,instructure/canvas-lms,fronteerio/canvas-lms,sdeleon28/canvas-lms,antoniuslin/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,AndranikMarkosyan/lms,kyroskoh/canvas-lms,utkarshx/canvas-lms,antoniuslin/canvas-lms,antoniuslin/canvas-lms,jay3126/canvas-lms,matematikk-mooc/canvas-lms,fronteerio/canvas-lms,sdeleon28/canvas-lms,redconfetti/canvas-lms,snehakachroo1/LSI_replica,jay3126/canvas-lms,instructure/canvas-lms,greyhwndz/canvas-lms,djbender/canvas-lms,sdeleon28/canvas-lms,sfu/canvas-lms,narigone/canvas,jay3126/canvas-lms,AndranikMarkosyan/lms,efrj/canvas-lms,narigone/canvas,leftplusrightllc/canvas-lms,skmezanul/canvas-lms,AndranikMarkosyan/lms,roxolan/canvas-lms,tgroshon/canvas-lms,tgroshon/canvas-lms,AndranikMarkosyan/lms,sfu/canvas-lms,HotChalk/canvas-lms,efrj/canvas-lms,juoni/canvas-lms,dgynn/canvas-lms,narigone/canvas,grahamb/canvas-lms,grahamb/canvas-lms,roxolan/canvas-lms,venturehive/canvas-lms,juoni/canvas-lms,snehakachroo1/LSI_replica,utkarshx/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,skmezanul/canvas-lms,greyhwndz/canvas-lms