Give a rough preview of TeX content

[gnprice]

This is a stripped-down early version, for the beta, of:

• TeX in the message list #46

Specifically, when we encounter TeX content (this Zulip feature: https://zulip.com/help/latex ), we'll want to show some usable approximation of it, pending the implementation of #46 with the full styling. This probably means showing the original TeX source — which conveniently is embedded inside the HTML — with perhaps some styling to indicate that it's TeX/LaTeX.

Handling these means parsing them in lib/model/content.dart, and then displaying them appropriately in lib/widgets/content.dart. In parsing, we should ignore all the parts of the HTML we won't yet be using — when we later pursue #46, we'll add parsing for those guided by our needs then.

This issue can also be seen as a special case of:

• Revise "unimplemented content" UX for general release #194

[gnprice]

A person who uses Zulip in a math-oriented community tried the beta and reported this unimplemented feature here:
https://chat.zulip.org/#narrow/stream/48-mobile/topic/TeX.20in.20Flutter.20app/near/1710610

This stripped-down version will be straightforward to implement, so I've gone ahead and bumped its milestone. (I'm more hesitant to prioritize the full implementation #46, because that will be a more substantial amount of work.) The one nontrivial question is how to style it.