Skip muted topics and streams on n key

[neiljp]

What does this PR do?

This is an adjusted version of the rebuild of #442 in #1239, so if merged will supersede both of those.

The motivation for this was upon considering the test adjustments which were absent in the draft updated PR (#1239), it seemed much more natural to migrate the appropriate function into the model itself. The main logic is therefore the same as #442, but the surrounding code is quite different, including tests.

I'm strongly considering merging this promptly, but I'd welcome a read-through, and testing by anyone who looked at #T1239.

This was discussed in #zulip-terminal > Skip muted topics and streams on 'n' #T442 #T1239 (may update topic name given this PR)

Tested?

• [x] Manually
• [x] Existing tests (adapted, if necessary)
• [x] New tests added (for any new behavior)
• [x] Passed linting & tests (each commit)

Notes & Questions

This explicitly adds what I believe is the current behavior of ordering by stream id and then topic name. As indicated in #1239, there is likely a lot of scope for improvement beyond that, assuming we want to mimic web app use of this feature.