Run `eslint` once in the root instead of in every package.

[atimmer]

Summary

We only need one invocation of ESLint for the whole repository.

Explanation

We currently run yarn lint in every package. This means that we need to define this in every package.json. We should remove those statements and instead have one yarn lint configuration inside the root package.json.

Technical decisions

• Combine all the --max-warnings statements into one --max-warnings flag in the root package.json.
• Change yarn lint to run eslint directly instead of running lerna run yarn lint.

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