react-express-mongodb: rename server to backend

The goal here is to keep the same structure than the other examples.
Also, considering that it has a "frontend" folder, the equivalent for
the server should be "backend".

Signed-off-by: Jérémie Drouet <jeremie.drouet@gmail.com>

react-express-mongodb/backend/config/config.js

@@ -0,0 +1,12 @@
+var env = process.env.NODE_ENV || 'development';
+
+if(env === 'development' || env === 'test'){
+
+    const config = require('./config.json')
+    let envConfig = config[env];
+    console.log(envConfig);
+
+    Object.keys(envConfig).forEach((key) => {
+        process.env[key] = envConfig[key]
+    })
+}