chore: add legacy service worker for PWA cleanup (#114)

Author: gablilli

frontend/static/sw.js

@@ -0,0 +1,13 @@
+// legacy sw.js to clean up old pwas
+self.addEventListener('install', () => {
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', event => {
+  event.waitUntil(
+    caches.keys().then(keys =>
+      Promise.all(keys.map(k => caches.delete(k)))
+    ).then(() => self.clients.claim())
+     .then(() => self.registration.unregister())
+  );
+});