Created by: sdemjanenko
This configuration is used in webpack's DefinePlugin which prefers keys
that look like process.env.REACT_APP_MY_ENV_VAR instead of a nested
object: https://webpack.js.org/plugins/define-plugin/
Prior to this change, webpack would replace all occurrences of
process.env with a JS object containing every key in the environment
that begins with REACT_APP.
The expected behavior is that a reference to
process.env.REACT_APP_MY_ENV_VAR is replaced with the value of just
that environment variable.