Update API_URL management (loaded from ENV)

Author: app-generator

env.sample

@@ -0,0 +1 @@
+REACT_APP_BACKEND_SERVER='https://YOUR_API_URL/';

src/config/constant.js

@@ -1 +1,8 @@
-export const API_SERVER = "http://localhost:5000/api/";
+let BACKEND_SERVER = null;
+if (process.env.REACT_APP_BACKEND_SERVER) {
+  BACKEND_SERVER = process.env.REACT_APP_BACKEND_SERVER;
+} else {
+  BACKEND_SERVER = "http://localhost:5000/api/";
+}
+
+export const API_SERVER = BACKEND_SERVER;