Open notifications at specific message, not just conversation #1565
Description
When you open a notification for some Zulip message, the app navigates to the message's conversation. This means either the newest messages in the conversation, or (after #80, by default) to your first unread message in the conversation.
It'd be good to go instead to the specific message you had a notification for.
When in the notification UI we have multiple messages from one conversation grouped together (something we currently do on Android but not yet iOS), that's one notification and opening it goes to a single place. For that let's use the earliest of those messages, because the user presumably wants to start from there before reading the later messages.
Rationale
Much of the time this doesn't make a difference. But when it does, the message from the notification is the one place in the conversation's history that we know the user is interested in seeing.
For example, the notification might have been an @-mention in the middle of a long-running conversation that you weren't previously reading. If we go to the first unread, that could be years and thousands of messages in the past, and not relevant.