Mark messages as read on scroll, configurably

[gnprice]

This Zulip feature: https://zulip.com/help/marking-messages-as-read#configure-whether-messages-are-automatically-marked-as-read

That means:

• When scrolling through messages that are unread, by default mark them as read with a request to the server.
• Control that behavior with a local setting "Mark messages as read on scroll", with three values: https://zulip.com/help/marking-messages-as-read#configure-whether-messages-are-automatically-marked-as-read
• Unlike most everything else in the Zulip data model, apply "latency compensation" when marking messages as read on scroll, meaning eagerly apply the change to the local UI rather than wait for the server to acknowledge it with an event.
  • Specifically, eagerly fade out the unread-message markers in the message list.

That local setting should go in the app's database on the device, from #13.

This should come after:

• Show unread markers in message list #79
• Open message list at first unread, rather than latest message #80

[gnprice]

Specifically, eagerly fade out the unread-message markers in the message list.

We might go further and eagerly update the model itself.

• The zulip-mobile RN app doesn't do this; the consequence is that it looks kind of glitchy as the "N unreads" banner at the top of the message list doesn't update or disappear in sync with the unread markers.
• I'm not actually sure what the web app does.
• The downside is that if we eagerly update the model, then it's no longer part of a coherent snapshot together with the rest of our model — we'd be giving up some of the consistency provided by the Zulip event system.

There are a couple of approaches we could take to that consistency question, but we should think through the consequences. So, leaving that out of scope for this issue.