unread list: Allow toggling of streams view (2nd approach)

[showell]

This is a somewhat more moderate approach to emulating the mobile unreads view on the main webapp while still playing nice with the old layout.

The first four commits, particularly the first one, should be ready to merge to master, regardless of where we go with the unreads view.

The fifth commit could also be merged, although it's mostly orthogonal to this effort, and we are still having conversation about making our search vs. filter terminology consistent.

The sixth commit is ready to test deploy, and it turns on unread-filtering for streams only.

The last couple commits are related to PMs, and I'm fine either way on test-deploying them. (There's still more work to do there, but the commits are coherent.)

Before test deploying, please ping me so we can communicate what's been done on this branch to czo users.

[timabbott]

This is exciting! I merged I think all of the early prep commits. A few notes:

• I'm going to test-deploy just the "remove more conversations" commit on czo to get some experience with it. My read is that we should likely merge it once we're sure it'd bug-free, since even if we added back a "more conversations" like concept, we'd probably want that to be a totally different thing that fetches more messages from the server. I also wanted to flag mobile: Add support for recent_conversations API. #11946 -- I think it'd be really valuable to integrate the data from that PR (which basically does what "more conversations" should have done), which would provide a 10x-50x better scrollback data set.

Didn't look much at the code for the rest due to merge conflicts from #11977.

[timabbott]

I just merged the "remove more conversations" commit as well, thanks @showell.

[showell]

The first commit here is the main one to test-deploy.

[timabbott]

OK, doing a test deploy of just that one.

[timabbott]

@showell just to check, should I include the WIP commits in the next test-deploy?

[zulipbot]

Heads up @showell, we just merged some commits that conflict with the changes your 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/master branch and resolve your pull request's merge conflicts accordingly.

[andersk]

The czo deploy of this is broken as of #12370; it throws TypeError: stream_data.in_home_view is not a function at left_sidebar.js:18.

[showell]

This is way too bit-rotted to resurrect. Also, I think "Recent Topics" is a better solution. Closing.