Debounce notifications so we don't orphan them

Creating/destroying notifications too quickly in testing on macOS would
result in them sticking around forever, requiring manual user dismissal.
We want to dismiss them for the user when we close or our window is
activated. So now we debounce() calls to our notifications code.

js/notifications.js

@@ -8,6 +8,7 @@
 /* global Signal: false */
 /* global storage: false */
 /* global Whisper: false */
+/* global _: false */
 
 // eslint-disable-next-line func-names
 (function() {
@@ -29,6 +30,13 @@
       this.on('remove', this.onRemove);
 
       this.lastNotification = null;
+
+      // Testing indicated that trying to create/destroy notifications too quickly
+      //   resulted in notifications that stuck around forever, requiring the user
+      //   to manually close them. This introduces a minimum amount of time between calls,
+      //   and batches up the quick successive update() calls we get from an incoming
+      //   read sync, which might have a number of messages referenced inside of it.
+      this.update = _.debounce(this.update, 1000);
     },
     onClick(conversationId) {
       const conversation = ConversationController.get(conversationId);