Replace cra with webpack

[emilioSp]

resolves #31, resolves #32

This PR contains:

• Remove useless files from cra (favicon, logo, etc…)
• Leaner index.html template in src
• Remove serviceWorker (we will use it when and if we need it)
• Add babel
• Add webpack@5
• Add jest
• Webpack 5 has introduced a breaking change: no more node.js polyfill (https://webpack.js.org/blog/2020-10-10-webpack-5-release/#automatic-nodejs-polyfills-removed). This is a problem because our dependency @apidevtools/json-schema-ref-parser is written for node.js. In order to make @apidevtools/json-schema-ref-parser work in the browser env we need to add several fallbacks in the webpack config.
• Change allowJs to true in ts-config because it is required by jest to parse test-utils.js
• Add identity-obj-proxy dev dep because it is required by jest config
• We have a problem with the bundle size (We could treat it with a different issue in the future)

[sebbalex]

@emilioSp can you fix conflict here? thanks