[app] Update frontend

* applied latest cra-template
* removed react-specific content

app/client/src/index.css

 body {
   margin: 0;
-  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen",
-    "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue",
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen',
+    'Ubuntu', 'Cantarell', 'Fira Sans', 'Droid Sans', 'Helvetica Neue',
     sans-serif;
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
 }
 
 code {
-  font-family: source-code-pro, Menlo, Monaco, Consolas, "Courier New",
+  font-family: source-code-pro, Menlo, Monaco, Consolas, 'Courier New',
     monospace;
 }

app/client/src/index.js

 import React from 'react';
 import ReactDOM from 'react-dom';
 import './index.css';
-import App from './Components/App';
+import App from './App';
 import * as serviceWorker from './serviceWorker';
 import '@fortawesome/fontawesome-free/css/all.min.css';
 import 'bootstrap-css-only/css/bootstrap.min.css';
 import 'mdbreact/dist/css/mdb.css';
 
-
-ReactDOM.render(<App />, document.getElementById('root'));
+ReactDOM.render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>,
+  document.getElementById('root')
+);
 
 // If you want your app to work offline and load faster, you can change
 // unregister() to register() below. Note this comes with some pitfalls.

app/client/src/serviceWorker.js

@@ -14,7 +14,7 @@ const isLocalhost = Boolean(
   window.location.hostname === 'localhost' ||
     // [::1] is the IPv6 localhost address.
     window.location.hostname === '[::1]' ||
-    // 127.0.0.1/8 is considered localhost for IPv4.
+    // 127.0.0.0/8 are considered localhost for IPv4.
     window.location.hostname.match(
       /^127(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}$/
     )
@@ -100,7 +100,9 @@ function registerValidSW(swUrl, config) {
 
 function checkValidServiceWorker(swUrl, config) {
   // Check if the service worker can be found. If it can't reload the page.
-  fetch(swUrl)
+  fetch(swUrl, {
+    headers: { 'Service-Worker': 'script' },
+  })
     .then(response => {
       // Ensure service worker exists, and that we really are getting a JS file.
       const contentType = response.headers.get('content-type');
@@ -128,8 +130,12 @@ function checkValidServiceWorker(swUrl, config) {
 
 export function unregister() {
   if ('serviceWorker' in navigator) {
-    navigator.serviceWorker.ready.then(registration => {
-      registration.unregister();
-    });
+    navigator.serviceWorker.ready
+      .then(registration => {
+        registration.unregister();
+      })
+      .catch(error => {
+        console.error(error.message);
+      });
   }
 }

app/client/src/setupTests.js

+// jest-dom adds custom jest matchers for asserting on DOM nodes.
+// allows you to do things like:
+// expect(element).toHaveTextContent(/react/i)
+// learn more: https://github.com/testing-library/jest-dom
+import '@testing-library/jest-dom/extend-expect';