issues #81: Display link previews beneath a message containing a link

[laonger]

Fixes #81
Remaining Issue:

1.	Due to the complexity of the page, many web pages cannot display the preview.
2.	The preview card requires scrolling the page to be displayed.

[tyreseluo]

You need to pass the ci first.

[laonger]

You need to pass the ci first.

done

[kevinaboos]

Thanks, this is a great start on this new feature!

Some questions:

1. do we actually need a dedicated cache type for link previews? Seems like a general map would work just fine, since it's a simple case.
2. The name "card cache" is confusing. What are "cards" in this context? Isn't it just a cache of link previews? Please change the term "card" accordingly throughout your whole PR.
3. Undo all formatting changes. All those unnecessary changes make it really hard for me to properly review this PR.
4. The widget structure of where you placed the LinkPreviewCard is ... unusual. See my comments about moving it into the Message widget's content subview within its body.

[tyreseluo]

One other issue, when there is more than one link, is the display of this card only one or, all of them, and when displaying too many cards, does it take up too much vertical space in the room_screen?

[tyreseluo]

1. Other than that, I think our MatrixRequest could have been simpler.

we just need url to fetch the description, title and preview image.

MatrixRequest::FetchLinkPreview { url } => {
    let _fetch_task = Handle::current().spawn(async move {
        let preview_service = url_preview::PreviewService::with_no_cache();
        match preview_service.generate_preview(&url).await {
            Ok(preview) => {
                log!("preview data: {:?}", preview);
            },
            Err(e) => {
                log!("Failed to fetch link preview card for {url}: {e}");
            }
        };
    });
}

when we get the preview data, then we store to chace or use Cx::post_action(Some); pass the data to the widget

2. We can use LinkPreview name to replace the Card name. also same to change the card_cache.

3. It seems we don't need to change much of the code in room_screen, just handle it in html_or_plaintext.

you can see this PR #390