Implement Topics and device groups

[paulincai]

I see two main uses:

1. A user may have multiple devices registered for Push. Subscribe them to a topic named with the userId so that I can send 1 single Notification to a Topic instead of pulling all tokens and send 1 by 1.
2. I may follow a hashtag or a subject in a social app (e.g. cars, tennis). I want to send those notifications without querying my DB for which users follow various interests. The point is that I can query Firebase for free while I also store the info with my Mongo.

https://firebase.google.com/docs/cloud-messaging/js/send-multiple