Handle no-topic topic ("general chat")

[gnprice]

The server will start allowing channel messages to have an empty topic at the API level, replacing the old "(no topic)" behavior. This should be displayed as something like "general chat" — with italics — in the UI.

The exact phrase to use for "general chat" is specified by the server in the API. For compatibility with old clients, the feature is gated by a flag the client sends. For details, see references below.

This is a feature planned for the upcoming Zulip Server 10.

Plan

We'll want a version of this soon, as work is proceeding on the server side. This will be a task for a member of the core team.

References

• #api design
• Initial server PR: Add support for empty topic names. zulip#32330
• Web PR: web: Add support for empty topic names. zulip#32963
• #design, for user/product motivation and UX choices
• Overall server/web issue: Rename "(no topic)" to "general chat" zulip#23291

[gnprice]

I've just sent #1266 as a refactor that will set us up to implement this, by distinguishing a TopicName type from plain String.

Other than that PR, I'd like to not drive this feature myself, in order to focus on other issues.

After that refactor, the places that will need to adapt to handle the empty topic differently from other topics can be found as the references to [TopicName.displayName]. There are about 9 such places. A couple of them will probably raise questions that should be discussed in the #design channel.

I believe the other pieces needed for the implementation are

• we'll need to track the new realm_empty_topic_display_name realm setting;
• for new servers, we'll need to remove the "(no topic)" (kNoTopicTopic) behavior and instead cheerfully send to the empty topic;
• we'll need to pass the empty_topic_name client capability, and the allow_empty_topic_name parameter to /api/get-messages and possibly other endpoints.