Open message list at first unread, rather than latest message

[gnprice]

When navigating to a message list, our default behavior (if there isn't a specific message we're asked to go to) should be to show the first unread message, if any, and the messages around it.

This means

• fetching around the first_unread anchor (https://zulip.com/api/get-messages), rather than latest;
• controlling the scroll position so that we show the first unread, if any, and the latest messages otherwise.

This should probably come after:

• Show unread markers in message list #79

A followup is:

• Mark messages as read on scroll, configurably #81