Handle Twitter previews

[gnprice]

This is a content feature that's probably largely historical (RIP Twitter), but exists in past messages.

Back when Twitter was Twitter and had a fairly open API, if you posted a link to a tweet, the server would turn that into a preview of the tweet. For example this message:

There's lots more examples in that #zulip channel on chat.zulip.org — we had a bot that would search for "zulip" and post new tweets it found. The previews stopped working in 2023-05, because Twitter (shortly before renaming to X) shut down most of its API for most users. We removed the feature from the server in https://github.com/zulip/zulip/pull/25816 .

We should support this feature, because it does exist in old messages which people may want to read. But it's a lower priority than most content features, because it doesn't exist in new messages.