Add PWA, service worker and Web Push

- Use new notification request/opt-in flow for push
- Implement unsubscribing
- Implement muting
- Implement emojis in title
- Add iOS specific PWA warning
- Don’t use websockets when web push is enabled
- Fix duplicate notifications
- Implement default web push setting
- Implement changing subscription type
- Implement web push subscription refresh
- Implement web push notification click

web/src/app/AccountApi.js

@@ -382,6 +382,10 @@ class AccountApi {
     setTimeout(() => this.runWorker(), delayMillis);
   }
 
+  stopWorker() {
+    clearTimeout(this.timer);
+  }
+
   async runWorker() {
     if (!session.token()) {
       return;