Upgrade to Babel 7 and update webpack

[jakub-g]

Tested in Chrome, Firefox, IE11, all works.

When built in production mode:

• main-legacy grows from 74k to 82k, main grows from 19k to 22k (AFAIU this is due to additional es7+ polyfills)
• runtime, import1, import2 stay the same size

One drawback is that due to debug: true in bundles.js there's dozens of warnings about @babel-polyfill:

[C:\git\webpack-esnext-boilerplate\node_modules\core-js\modules\es6.typed.int8-array.js] `import '@babel/polyfill'` was not found.

[C:\git\webpack-esnext-boilerplate\node_modules\core-js\modules\es6.string.sup.js] `import '@babel/polyfill'` was not found.

[C:\git\webpack-esnext-boilerplate\node_modules\core-js\modules\es6.typed.array-buffer.js] `import '@babel/polyfill'` was not found.
...

According to https://github.com/babel/babel-loader/issues/657 this is a non-issue and not easily fixable.

What we could do here is to to disable debug but that would be a shame. Any suggestions what to do with this @philipwalton?

[philipwalton]

I'm fine with the verbosity in debug mode. Since this is a demo, I'd rather people see the output than have it be behind-the-scenes magic.