msglist: Control scroll position on renarrowing for bulk message moves

[chrisbobbe]

When you're in a narrow and messages are moved in bulk, we renarrow the message list page to the messages' destination. (#1008 would remove an uncommon case where we do this undesirably.)

When we renarrow, we should try to keep the messages the user was reading in view, preserving their scroll position when we can. Currently, we just open the new narrow at its latest messages. (This might change with #80, but that's still not the behavior described here.)

To fix, we can choose some onscreen message that was moved and try to maintain its scroll position across the move. For how we might choose that message, see #1008.

Marking as post-launch because we don't do this in zulip-mobile.

This is different from #83; that issue doesn't address renarrowing.