content: Support Zulip content outside messages (even outside per-account contexts)

[chrisbobbe]

The web app supports Zulip Markdown rendering in contexts other than just message content:

See for example realm_description from the server settings on CZO, shown on the login page:

And see rendered_description on stream objects in the /register response, shown in stream settings. This screenshot shows links (like [text](link)) being rendered in some stream descriptions:

This kind of thing would be easy to support in our app, except that some of the widgets in lib/widgets/content.dart have grown dependencies on data that won't be available in these other contexts. For example, MessageImage uses RealmContentNetworkImage, which requires a PerAccountStoreWidget ancestor. It also links to the lightbox, which is also per-account and additionally pulls data about the message's sender.

Currently, the following single line of code (atop #281) actually succeeds in visually reproducing CZO's realm description:

BlockContentList(nodes: parseContent(widget.serverSettings.realmDescription).nodes)

But the links don't work; an error prints to the console when I tap one of them.

[chrisbobbe]

So, we need the content widgets to work even when their message- and account-data dependencies are absent. Ideally, we would have a way to enforce that such strict dependencies aren't reintroduced.

[chrisbobbe]

I noticed that this repo's a-content label has this description: "Parsing and rendering message contents"

Should we generalize that to something like "Parsing and rendering Zulip content, like in messages"?

[gnprice]

Should we generalize that

Sure, done: "Parsing and rendering Zulip HTML content, notably message contents".

[alya]

However we end up presenting stream descriptions, we'll probably want to show descriptions for views the same way as well. Web app issue: zulip/zulip#29769.