C2LC-277: Use ESLint to check indentation

Description

Add rules to our eslintConfig to check indentation.

I did a little experimenting this morning and the following seems like a good starting place:

"indent": ["error", 4, {
    "CallExpression": {"arguments": "off"},
    "SwitchCase": 1
}],

See: https://eslint.org/docs/rules/indent

src/serviceWorker.js was generated by create-react-app and doesn’t match our 4 space indentation. We may as well update its indentation as part of this work.

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