Add CSS for new hl_lines feature of code blocks if needed

[timabbott]

We're close to merging zulip/zulip#16984, which will add support for the pygments hl_lines feature (being able to highlight lines in a markdown code block). It's probably OK to ignore the composition/local echo parts of this new feature and just expect to get that as part of future syncing code from the webapp project.

We should test how that new feature looks in the mobile app, and if it's ugly, fix it. (The original PR has night them styling that may be useful as well).

As a sidenote, is it useful to document features like this in the API changelog?

[chrisbobbe]

As a sidenote, is it useful to document features like this in the API changelog?

I think it might be helpful to note any new HTML classes; there was once an odd bug described here, fixed in #4328. As part of that PR we added to our style doc:

HTML

Avoid classes that the server might use in messages: In our own
HTML in the webview, we avoid using any class names which appear in
message content as rendered by the server.

There isn't a single comprehensive list of these. Most of them can be
found in static/styles/rendered_markdown.css in zulip/zulip, which
is where the webapp styles the message content. In addition to names
used in current Zulip Server versions, we need to avoid those used in
the past -- not only because some servers will still be on old
versions, but also because old messages generally aren't re-rendered
into HTML even after the rendering logic changes.

(In principle we should take steps to avoid future names, too. We
don't worry about that for now, because this hasn't often been a problem.)

See chat discussion for further rationale.

Still, that comment helpfully points to static/styles/rendered_markdown.css, so we won't be totally lost if they're missing from the changelog; also, it's not a really common issue. Maybe a note in the changelog would be helpful if it's about a new HTML class that's a really commonly used term, so more likely to collide with something we're using.