unread: Do full rerender of recent view on mark unread.
unread: Do full rerender of recent view on mark unread.

This matches the current bulk-operation behavior of the message lists,
and avoids spending an enormous amount of time adjusting unread counts
once per message in a loop over thousands of messages.

Also add some comments on adjacent code paths.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip