Refactor env vars to use process.env everywhere

[cray0000]

On server side use dotenv to load everything into process.env. On client side write a babel plugin which will transform all process.env variables from runtime env and from .env file, but only the ones which have a prefix PUBLIC_*. It might function similar to https://github.com/brysgo/babel-plugin-inline-dotenv which generates process && process.env ...