Swift.org preview card missing or unsuitable for embedding.

[timsneath]

Describe the bug

See the attached URL in desktop Safari:
https://www.reddit.com/r/swift/comments/1efy1xg/announcing_swift_homomorphic_encryption/

Screenshot below -- note that it is a large, blurry Swift icon.

Expected behavior

Other pages have far nicer, custom-designed preview cards. For example:
https://www.reddit.com/r/swift/comments/1eqbvrh/fatbobmans_swift_weekly_044/

Screenshots

[timsneath]

These two sites provides preview tests for how our social card shows up:
https://socialsharepreview.com/?url=https://swift.org
https://www.opengraph.xyz/url/https%3A%2F%2Fswift.org

[kaishin]

We can tackle this as part of the site redesign as this might require new assets. Ideally we should be using dynamic preview cards but I am not familiar with Jekyll's capabilities in that area.