TRACKING: Improve algorithm for 'next unread topic' (`n`)

[neiljp]

For a small number of unreads, this feature works reasonably well, and #1245 migrated older work to skip muted topics and streams.

However, as discussed in the related topic and elsewhere, the current algorithm for next-unread-topic, via the n hotkey, is lacking in various ways.

These include:

• [x] there is no context based on the current narrow/message, but rather separate state stored in the model; this leads to various behavior, including typically jumping out of the current stream, even if it still has unread topics: one must fully read that stream via this method first [fixed in #1356]
• [x] the ordering of moving between streams is based on stream id, not stream name; this causes the order of streams being fully read to jump around and not match the order in the left panel (if visible) [fixed in #1412]
• [ ] #1582

These elements are obvious deviations from the behavior of the web app, but others may be present.