Open a message list in the middle of history, when requested

[gnprice]

When the user takes an action that should navigate to a particular "anchor" message (like through #73 internal links, #76 deep links, or a notification), we should take them straight to that message, rather than to the latest message or first unread message in the narrow.

This is a feature the zulip-mobile RN app lacks: zulip/zulip-mobile#3604 . (That's for internal links, but the app doesn't have it for notifications either — see narrowToNotification which passes no anchor to doNarrow — and doesn't have deep links.)

There are two parts to this:

• Fetch the messages around the desired anchor, rather than around the latest message or the first unread.
• Control the scroll position so that we show the desired anchor message.

This has a lot in common with:

• Open message list at first unread, rather than latest message #80

This issue has fewer prerequisites technically than #80 — no need for #79 first. On the other hand it has more prerequisites before there's a UX context where it makes sense: it'd need either #73, #76, or notifications, all of which are more complex than #79.