Skip muted streams and topics using `n` (from #442)

[neiljp]

What does this PR do?

This is a WIP taking the last commit of #442, rebasing, reformatting and correcting for how that PR treats is_muted_topic as also incorporating if the stream is muted (which the model method we have now doesn't do).

This was brought up again recently in #zulip-terminal > ZT impressions by @rht but I'm continuing the discussion in #zulip-terminal > Skip muted topics and streams on 'n' #T442 #T1239.

I tested this in explore mode and toggled muted streams and topics. For example, where it starts on #general (id=2) for me, and will skip #backend (id=3) next if muted, and then also skips #test here just fine.

Tested?

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

Commit flow

The flow is very WIP; this is just for functional testing and broad feedback. I've reformatted and it should be functional, but I've not adapted the tests from #442 so far.

Notes & Questions

Would appreciate feedback from anyone who uses the n shortcut regularly.

Refactor in last commit improves clarity?

This doesn't fix various other factors regarding this feature, including:

• order of unreads is stream via stream id, and topic alphabetically, neither of which match the left panel list order (which is likely desirable)
• I've not dug into if the unreads get updated cleanly when messages get read or new messages are received

[rht]

I have tested this and it works great.

[neiljp]

Closing in favor of #1245, which is now merged 🎉

[zulipbot]

Heads up @neiljp, we just merged some commits that conflict with the changes you made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.