Fallback to a card block for pasted URLs that don't return oEmbed data

[claviska]

Currently, Postleaf looks for an oEmbed service when a URL gets pasted into the editor or the embed tool. If the site doesn't support oEmbed, the URLs just gets inserted as-is.

There probably aren't a lot of cases where a user pastes a URL into the editor and actually wants the URL to as-is. Using Discourse today, I noticed they produce a card block (onebox) for non-oEmbed URLs:

This would be a great enhancement for Postleaf. As with oEmbed providers, the theme should be able to customize their appearance to match the surrounding content.

Possible candidates to replace the existing oEmbed lib:

• https://www.npmjs.com/package/metaphor

[claviska]

Here's what they look like:

Pasting or embedding a URL in the editor (or inserting a URL in quick post) will generate one of these previews under the following conditions:

• The URL isn't an image (it will be inserted as an image if it is)
• The URL doesn't support oEmbed

Note: card styles are 100% configurable in the theme.

[claviska]

If you want to try this in master, make sure to git pull the default theme to get the necessary styles.