Handle message_embed website previews

[gnprice]

I.e., this Zulip feature:
https://zulip.com/help/image-video-and-website-previews#configure-whether-website-previews-are-shown
which we've also called "inline URL preview".

The HTML elements for these look like <div class="message_embed">. Example message here:
https://chat.zulip.org/#narrow/stream/101-design/topic/auto-linkification.20of.20issues/near/193181

On chat.zulip.org these are scarce because it's a realm-level setting which we turned off after a brief period enabled, so it's easy for us to forget about this feature. But on realms that have the setting turned on, they'll naturally be much more common.

These appeared in the results of the survey #917 / #190 of unimplemented content features. Thanks to @andersk for pointing out (#965 (comment)) that this is a current feature.