Using `n` should move between topics by recency

When cycling through unread topics using n, this is not always sorted by the order in the topic list, ie. by most recent new message to a topic.

For discussion on related issues, see the topic in # zulip-terminal starting Improve algorithm for 'next unread topic' (n) #T1333 #T1356.

This behavior should be noticeable compared to when using the web app.

Using 'Explore' mode may be useful to ensure you don't keep marking messages lots of messages read during manual testing.