Fix the way we're prerfetching i18n for dayjs + VS locale

[pkarw]

What is the motivation for adding / enhancing this feature?

Currently in the production mode we're prefetching all possible locales:
https://www.dropbox.com/s/jktt4j5123g9cc0/Screenshot%202019-09-05%2021.05.07.png?dl=0

We shouldn't. It's because:

• https://github.com/DivanteLtd/vue-storefront/blob/d91ae485fbb12abe84c76a8bf04ea51299a14395/core/i18n/index.ts#L47
• https://github.com/DivanteLtd/vue-storefront/blob/d91ae485fbb12abe84c76a8bf04ea51299a14395/core/i18n/index.ts#L31

are expanded by webpack (glob) to all possible locales - then prefetched

[pkarw]

This occurs only in production mode. Potential fix:
https://github.com/jantimon/resource-hints-webpack-plugin

plugins: [
  new HtmlWebpackPlugin({
    prefetch: ['**/*.*'],
    preload: ['**/*.*']
  }),
  new ResourceHintWebpackPlugin()
]