Support Yarn package manager

[getspooky]

React Boilerplate

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Description

The main reason why support Yarn package manager in react-boilerplate. In the case of npm, when we need to deploy the project on different machines, the versions of installed packages can be different.

1. can install packages from the local cache
2. strongly binds package versions
3. allows parallel packages installation
4. Yarn is its stability

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owners to license your work under the terms of the MIT License.

[coveralls]

Coverage remained the same at 100.0% when pulling 4eca6d6 on getspooky:dev into e9ed2b6 on react-boilerplate:dev.

[gretzky]

Could you write in the PR description the purpose of this change / why this change is necessary?

[Can-Sahin]

Edit: Sorry I opening this back as a result of not merging the #2935 if that means anything;)