Remove notifications when messages are read, on iOS

[chrisbobbe]

We have this feature on Android (that was #341) and should implement it on iOS.

It wasn't implemented in the legacy iOS app (zulip/zulip-mobile#3119), so it hasn't been a near-term priority. But it came up in user feedback yesterday:

#mobile > clearing notifications on message read @ 💬

I notice that on Telegram when I open a message, even on desktop, the notification gets cleared on mobile. It would be awesome if zulip also did that. It's a point of feedback we get in our group because people have notifications that they've already read. Personally, I just clear all zulip notifications and check my inbox, but having the notifications cleared on read would be much smoother in my opinion.

References

• Prerequisite: ios notif: Control the showing of notifications #1265