React client

[RayhanTabase]

In this pull request:

• Created a new React app called 'hello-react-front-end'
• Created the App component with react-router.
• Created the Greeting component that will display a greeting. Set it up as a route in your App component.
• Created a store, an action, and a reducer that will connect to your API endpoint to get the random greeting.
• Display the random greeting in your Greeting component.