Extra recipient header when topic changes case

[gnprice]

If you visit the thread #translation > Missing string on chat.zulip.org, around this message (the first one in today's thread): https://chat.zulip.org/#narrow/stream/58-translation/topic/Missing.20string/near/1828362

you'll find it gets its own recipient header, even in a topic narrow, even though there are previous messages. Specifically there's a recipient header "missing string" for the older messages (which are from 2021), and another one "Missing string" for that message and the ones after it.

In general Zulip topics are case-insensitive. Specifically, if you look at that conversation in Zulip web, there's just one recipient header; it reflects the older messages, so "missing string". So we should do the same.