Notifications: Add Firebase ID storage

[arghgr]

In order to send push notifications via Firebase, we need to store a randomized ID for each device that approves our request to send them push notifications. We can store these IDs either in Firebase itself or on our own server. We're going to do the latter (storage on the server), which is the typical way to do this, and would also be faster because we wouldn't have to fetch all the IDs from Firebase before sending the push notification.