Hide topic field on channels restricted to "general chat" only

On the web version, the topic box is hidden on channels where topics are disabled (i.e. general chat only). On this mobile client, the topic field is available, and setting the value results in an error.

I think it's much more intuitive to hide the topic box on mobile in channels that are general chat only.

References

• Prerequisite issue: #1604
• Help center on this setting: https://zulip.com/help/general-chat-channels
• API docs: see topics_policy under https://zulip.com/api/get-events#stream-create