unread: Indicate which streams and topics have unread @-mentions.

[jai2201]

Fixes: #21637

Screenshots and screen captures:

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[timabbott]

I did some rewriting of this, primarily inspired by the insight that the reverse_lookup function just needed a small tweak to return the bucket_key rather than the bucket to support this without larger changes to the Bucketer class. I also eliminated some code duplication in stream_list.js, and fixed case-sensitivity issues with topics and undefined being incorrectly added to the set of topics with unread mentions.

I'd appreciate your tweaking the tests so that they would fail if we didn't handle case-sensitivity correctly.

There's a TODO I added that I'd like you to investigate.

[timabbott]

Also can you fix the node test? Might be best done in a new commit we plan to squash, just so it's easy for me to see your changes.

[jai2201]

@timabbott , I added a prep commit above and rebased the other commits accordingly and made each individual commit pass the CI, can you please take a look at them now?

[timabbott]

I'm going to test-deploy this on chat.zulip.org.

Can you add a proper commit message to the new prep commit explaining why it's correct? Specifically, what code fills in the unread count already? And does the history suggest why this function existed if it's a noop? git log -p / git log -S may help explore that.

[jai2201]

Ahh, Thanks a lot @timabbott for suggesting to look in the history why this function was introduced if it's a noop;

I looked through the commits in history and have found something tricky which makes this prep commit buggy;
Here's the link of the commit which introduces this piece of code.

[jai2201]

@timabbott , I've updated the PR with completing the TODO added and made the node tests pass for each individual commit;

Left some comments above for you to have a look;
Would like to request for a re-test deployment of this PR on CZO.

[timabbott]

I'm going to merge this, but I think it'd be better to change this test to use the real unread.js module; it's a data module, so it should be easy to setup with a bit of data for the relevant streams rather than mocking it.

[timabbott]

Merged, thanks for doing this @jai2201! Hopefully you've learned a bit about how to do simple but efficient data structures through this project :)