[WIP] Setup yarn offline

[willdurand]

Fix mozilla/addons#10714

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• setup offline mode
• use offline mode everywhere

[codecov-io]

Codecov Report

Merging #5896 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    mozilla/addons-frontend#5896      +/-   ##
==========================================
+ Coverage   97.72%   97.77%   +0.04%     
==========================================
  Files         229      229              
  Lines        5858     5987     +129     
  Branches     1123     1171      +48     
==========================================
+ Hits         5725     5854     +129     
  Misses        118      118              
  Partials       15       15

Impacted Files	Coverage Δ
src/disco/components/Addon/index.js	100% <0%> (ø)	⬆️
src/amo/components/Addon/index.js	100% <0%> (ø)	⬆️

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[willdurand]

So I can't make it work, it says:

error Can't make a request in offline mode ("https://registry.yarnpkg.com/babel-polyfill/-/babel-polyfill-6.26.0.tgz")

The package is checked in though, so I don't understand...