Replace App.getInitialProps with custom headers() in next.config.js

[viktor-yakubiv]

Having getInitialProps in the custom App disables static page optimisation for all pages, if I understand. We use this method only to add a custom header.

This is possible to do adding custom headers() function to the next.config.js. The function adds HTTP headers to the URL. It's also possible to have custom headers per page.

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This could be related to #310