Add eslint

[magopian]

Fixes #34

• Add eslint to project (let's use the one bundled by default with CRA)
• Add Airbnb eslint config
• Add prettier-eslint, change package.json scripts accordingly
• Ensure the CI process properly lints

[magopian]

We might not want airbnb eslint config after all:

• CRA's creator isn't really happy with airbnb's config: see CRA and Airbnb require conflicting versions of eslint-plugin-jsx-a11y facebook/create-react-app#3540 (comment)
• there's some dependency version issues between the version of eslint that comes with CRA, and the versions supported by eslint-config-airbnb: jsx-a11y/href-no-hash removal airbnb/javascript#1474

[mythmon]

I'd suggest eslint's recommended configuration combined with eslint-prettier. The recommended config handles most of correctness rules, and prettier handles formatting. It's not super opinionated like airbnb, but still has a decent set of rules. This is what I did in mozjexl

[rehandalal]

wheee! r+