notifications: Support "badge count" on iOS

[chrisbobbe]

Counterpart to zulip/zulip-mobile#438. Quoting from there:

We want to show a badge on the app's icon on the iOS home screen when the user has notifications for messages they haven't read.

I believe what has stalled that issue is a server-side bug with the payloads we're constructing; see discussion: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/iOS.20badge.20count/near/938530

Once servers send the right badge number, we might then need client-side logic (or logic in the push bouncer?) to aggregate those numbers, because the app might be logged into different accounts on different servers. E.g., for logged-in accounts A and B, if there are 3 unread notifications for A, and 2 for B, presumably we should show 5 as the badge count.