Make it easier to pick (or not to pick) a topic

[alya]

As discussed on CZO, users (especially those who are new to Zulip) sometimes find it challenging to pick a topic for their message. We should (a) do what we can to make picking a topic feel easy, and (b) make it feel fine not to enter a topic for organizations that enable that option.

This is an umbrella issue for changes working towards these goals. Specific changes we should make:

• Rename "(no topic)" to "general chat" #23291
• Change topic placeholder text for organizations that allow no-topic messages #23294 (to be marked as "help wanted" when Rename "(no topic)" to "general chat" #23291 is completed)
• Add hint text to topic typeahead dropdown #23295
• Add a new topic indicator to the compose box topic typeahead #23296
• Make topic name for messages sent without a topic customizable  #23292 (higher complexity / lower priority)

[zulipbot]

Hello @zulip/server-onboarding members, this issue was labeled with the "area: onboarding" label, so you may want to check it out!

[dna-witch]

@zulipbot claim

[zulipbot]

Hello @dna-witch!

Thanks for your interest in Zulip! You have attempted to claim an issue without the label "help wanted". You can only claim and submit pull requests for issues with the help wanted label.

If this is your first time here, we recommend reading our guide for new contributors before getting started.

[alya]

Closing in favor of #23291 at this point.