Handle legacy message_inline_ref Dropbox previews

[gnprice]

Some messages' content can have an HTML element like <div class="message_inline_ref">.

This is a legacy Zulip feature; it's still live in current servers, but looks broken in current web clients. I think we hadn't thought about it in a number of years, until it was discovered in the #917 / #190 survey of content features not yet implemented in this new mobile app. The next time we spend effort on it on the server or web side, it'll probably involve eliminating this feature from the HTML.

Given that context, supporting these is a low priority. Filing this issue so we can track it, though.

Chat thread: https://chat.zulip.org/#narrow/stream/9-issues/topic/message_inline_ref.20dropbox.20links/near/1934079

Original notes (with a link fixed) by @PIG208 in #921:

Added here. This pretty much a dropbox-specific link preview (this line of code is 10 years old) for non-images. This still can be generated by the server. (example)