Created by: fson
Move the ESLint configuration to a separate package eslint-config-react-app. This allows using the same configuration in projects not using CRA. It also makes the amount of configuration in an ejected project smaller: the starter ESLint config of an ejected the project will be just {"extends": "react-app"}.
Test plan:
[x] Check that linting errors are shown correctly when running npm start.
[x] Check that it also works end to end. (Tested with a local npm server.)
(NB: the e2e test will break at the moment because the package hasn't been published yet, and it tries to install it from npm. However, I tested by publishing it to a local npm registry with sinopia, and it worked. I think we might want to come up with a way to end-to-end test all the packages together without having to manually pack all of them in the test script. But for now pulling the linter config from npm is probably good enough?)