[notification] Debounce new mail notification sound

Summary:
We don't want to overwhelm a user with a bunch of bings and bongs when
they open their laptop after a long weekend. This diff takes a
relatively simple approach by debouncing the notification sounds every 5
seconds.

Test Plan: Run locally, make sure we still get notified but not too much.

Reviewers: juan, evan

Reviewed By: evan

Differential Revision: https://phab.nylas.com/D3841

internal_packages/unread-notifications/lib/main.es6

@@ -180,7 +180,8 @@ export class Notifier {
         }
         if (!this.hasScheduledNotify) {
           if (NylasEnv.config.get("core.notifications.sounds")) {
-            SoundRegistry.playSound('new-mail');
+            this._playNewMailSound = this._playNewMailSound || _.debounce(() => SoundRegistry.playSound('new-mail'), 5000, true);
+            this._playNewMailSound();
           }
           this._notifyMessages();
         }