msglist: For bulk moves with `PropagateMode.changeLater`, don't renarrow if user was reading earlier messages

[chrisbobbe]

When messages are moved out of the current narrow with PropagateMode.changeLater, we always follow the move by renarrowing MessageListPage. If the user is scrolled up to much earlier messages than the ones that were moved, the context change will be unhelpful and confusing.

To fix, we can guess which onscreen message the user is reading and only renarrow if that message was moved. We can't always guess correctly, and that's fine. (When more than one message is visible, the user's eyes could be on any of them.) But a simple algorithm would be to choose the first message that's at least partly shown in the viewport. If screen-reader software is active and a message is selected, that message is probably a better choice.

Marking post-launch because zulip-mobile doesn't do this.