Add polling and automatic redirection to /start on email confirmation (#25013)

app/javascript/packs/sign_up.js

@@ -0,0 +1,15 @@
+import './public-path';
+import ready from '../mastodon/ready';
+import axios from 'axios';
+
+ready(() => {
+  setInterval(() => {
+    axios.get('/api/v1/emails/check_confirmation').then((response) => {
+      if (response.data) {
+        window.location = '/start';
+      }
+    }).catch(error => {
+      console.error(error);
+    });
+  }, 5000);
+});