react-express-mysql: split backend code

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

react-express-mysql/backend/src/config.js

@@ -0,0 +1,7 @@
+// Constants
+module.exports = {
+  port: process.env.PORT || 8080
+  // if you're not using docker-compose for local development, this will default to 8080
+  // to prevent non-root permission problems with 80. Dockerfile is set to make this 80
+  // because containers don't have that issue :)
+};