ios notifs: Support tapping a notification to open the conversation

[chrisbobbe]

Like #123, but for iOS.

When you tap a new-message notification, we should open the message's conversation.

[chrisbobbe]

Beta feedback requesting this: https://chat.zulip.org/#narrow/channel/48-mobile/topic/beta.20feedback.3A.20.28ios.29.20notifications.20should.20open.20conversation/near/2067876