feat(service-worker): add unregister script and update index.html

- Introduced a new script to unregister service workers, enhancing control over service worker lifecycle.
- Updated index.html to include the unregister script, ensuring it is loaded for proper service worker management.

worklenz-frontend/public/unregister-sw.js

@@ -0,0 +1,7 @@
+if ('serviceWorker' in navigator) {
+  navigator.serviceWorker.getRegistrations().then(function(registrations) {
+    for(let registration of registrations) {
+      registration.unregister();
+    }
+  });
+}