Provide the browser notification #261

At this moment, we're providing the Slack notification for deployments and reviews, but it is possible to provide the browser notification.

The notification logic is determined by the kind (deployment or review). The browser should notify only the user who triggers the deployment for the' deployment' event. For the review event, it should notify the requester when the review is responded to, but it should notify the reviewer when the review is requested.

Implement

We should implement two parts for the browser notification like the below:

1. Provide the setting to configure the notification whether to receive or not.
2. Notify the user when the event is fired.