Dedupe sending notification token to server

[gnprice]

In my initial implementation of #122, I plan to have the app just tell the server its notification token again and again on each launch.

As a follow-up, we should have it instead remember when it's already successfully told the server its token, and then not tell it again. We do this in zulip-mobile.

Actually, there's a refinement to make. Google's docs recommend that the client should repeat itself to the server, but only after a month: https://firebase.google.com/docs/cloud-messaging/manage-tokens#ensuring-registration-token-freshness That then allows the server to conclude when a token hasn't been re-registered in, say, two months, that it's stale — that the client has gone away — and Google recommends that the server stop sending notifications to it in that case. (I don't recall seeing that recommendation the last time I studied these docs, several years ago. Not sure if Apple also recommends something similar these days.)