Add push notifications w/next-pwa

[hakanayata]

This commit addresses issue #618, and implements push notifications using next-pwa and web-push.

To test this feature please follow these steps:

1. Generate VAPID keys using yarn web-push generate-vapid-keys [--json] and

2. Add your generated keys and email to the .env file in the following format:

• [email protected]
• WEB_PUSH_PRIVATE_KEY=your-vapid-private-key
• NEXT_PUBLIC_WEB_PUSH_PUBLIC_KEY=your-vapid-public-key

NOTE: To enable the app to run as a PWA, the site.webmanifest file has been renamed to manifest.json.