Handle topic links

[gnprice]

This is the feature expressed by the topic_links field on message objects (https://zulip.com/api/get-messages).

In web, this is shown as a "link out" icon in the recipient header. It's the first icon after the topic name in this screenshot (of this message):

Chat thread: #mobile > Topic linkifiers @ 💬

[gnprice]

@aditya-cherukuru Posting AI slop wastes maintainers' time. If you'd like to contribute to Zulip, please take the time to read our contributing guide, linked from the README file.