Add support for sending notifications to Apple private email relay service

When users create account with Apple ID (icloud) and choose an option to hide their email, all you get is email with @privaterelay.appleid.com. As of now, our users that have created account with this option are not receiving mail notifications from info@featurebase.app. To confirm this, I've created a new icloud account as a brand new user and confirmed that the users indeed don't receive notifications from Featurebase on their icloud email.

If you want to send an email to those users, or automated email notification, Apple requires you to first make some configuration on your domain (in this case featurebase.app)

Useful links:
https://developer.apple.com/documentation/sign_in_with_apple/sign_in_with_apple_js/communicating_using_the_private_email_relay_service

https://developer.apple.com/help/account/configure-app-capabilities/configure-private-email-relay-service/

If you need any more information, feel free to contact me.