Fcm tokens are not properly updated

[verbotenj]

In the function AddNewFcmTokens, we are only adding new tokens. We don't remove tokens.
https://github.com/blinklabs-io/snek/blob/main/output/push/push.go#L192

The expected way is to add/remove tokens when they are added or removed through API.

[verbotenj]

Fixed #159