Integrate Firebase push notifications for iOS devices

[seanstrom]

Feature Task Issue

Description

• We've implemented Firebase support in the past for Android, though we have not integrated Firebase for iOS yet. Since we'll be adding support for Firebase on Android, it would be nice to integrate iOS devices too so we can send push notifications from a single source, and possibly retrieve notifications from Firebase.

Acceptance Criteria

• The mobile app should be able to request push notification permissions from the user,
  • And request a push notification token from Firebase
  • And we should be able send a test push notification from Firebase Console to the iOS device using that push notification token.

Notes

• Currently, iOS devices use a APNS token for subscribing to chat push notifications, but it would also be possible to use Firebase for communicating those chat push notifications too. Though for the moment, we'll only register the iOS device to Firebase, and still use APNS token directly with our Status push notification servers.