Make one place config and different files for changing environments

[Ozgur Erdem]

Currently there are two places for configuration: ./config.js and ./src/config/config.json. There should be only one place for config.

Also in config.json, changes based on environments are listed in one file. It is better to seperate them to different files and specifying where it runs with NODE_ENV environment value. This development style explained in this post https://stackoverflow.com/a/36695742/153570