* Bootstrap with Yarn if we can

* Update test scripts

* Check OS and npm concurrency ability

* Windows support

* Update bootstrap.js

* Install yarn before bootstrap

* Update dependencies

* Use published version of eslint-config-react-app for development

We dogfood on our eslint-config-react-app by using it as our ESLint config,
but installing it from the local folder causes problems when its peerDependencies
are updated, because npm won't update it unless the version number changes.

Use the published version for dogfooding so it can be installed from npm.