Upgrade sass-loader to 7.0.1 & stop bringing node-sass bin to each user

[Hypnosphi]

Issue: #3425

This update allows us to stop making users download a large node-sass binary even if they don't use sass/scss at all.

See also: webpack-contrib/sass-loader#532

[ndelangen]

I'm OK with this..

[codecov]

Codecov Report

Merging #3467 into master will decrease coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3467      +/-   ##
==========================================
- Coverage   37.56%   37.14%   -0.43%     
==========================================
  Files         459      465       +6     
  Lines       10235    10320      +85     
  Branches      924      934      +10     
==========================================
- Hits         3845     3833      -12     
- Misses       5816     5937     +121     
+ Partials      574      550      -24

Impacted Files	Coverage Δ
addons/graphql/src/components/FullScreen/index.js	0% <0%> (-80%)	⬇️
addons/graphql/src/components/FullScreen/style.js	0% <0%> (-66.67%)	⬇️
addons/a11y/src/components/Report/index.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/element_check.js	67.64% <0%> (ø)	⬆️
addons/storyshots/src/rn/loader.js	53.33% <0%> (ø)	⬆️
addons/knobs/src/components/types/Boolean.js	11.62% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/index.js	29.33% <0%> (ø)	⬆️
addons/knobs/src/angular/helpers.js	76.76% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/search_box.js	23.52% <0%> (ø)	⬆️
addons/knobs/src/mithril/WrapStory.js	0% <0%> (ø)	⬆️
... and 82 more

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