Mark a thread as unread in the threads list due to activity.

[clokep]

Follow-on to #9723 to also show the thread as unread in the threads list:

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Mark a thread as unread in the threads list due to activity. (#9738).

[clokep]

(Should probably have the same changelog as #9723, but I'm not sure how to do that?)

[germain-gg]

That looks good to me!
Tests are failing, but from what I gather if you get up to date with the upstream develop that should pass. There's nothing that should cause that failure

[clokep]

We'll want to wait to merge this until after the reversion of #9723 lands (see #9745) and we re-land that with fixes.

[clokep]

Going to close this and rebase it on a reworked #9723.