WordCamp Sites: Open Graph -- add default image fallback when there is no image in the post.

[jeherve]

See discussion here: ​https://wordpress.slack.com/archives/events/p1453392084003279

We already have a default Open Graph Image Meta tag added on the home page, when you haven't set any Site Logo, or Site Icon on your site.

It would be nice to have a similar feature on single posts, when no image can be found in the post (none was inserted, and no featured image was defined).

The attached patch introduces 4 levels of fallback:

1. Site Logo -- ​http://jetpack.me/support/site-logo/
2. Site Icon -- ​https://make.wordpress.org/core/2015/07/27/site-icon/
3. Header Image -- ​https://codex.wordpress.org/Custom_Headers
4. Global default OG image -- that's already defined in jetpack-tweaks.php, it's a WordPress logo: ​https://s.w.org/images/backgrounds/wordpress-bg-medblue.png

One concern from our Slack discussion was the following:

Though many header images are PNGs which render transparency kinda weird on shares (especially twitter)

-- @marktimemedia

[jeherve]

Open Graph: add default image fallback when there is no image in the post.

[jeherve]

Open Graph: add default image fallback when there is no image in the post, proper diff

[jeherve]

Please ignore the first patch, default-fallback-image.diff​, I messed up when creating that first diff.

[iandunn]

Related #1019, #1261

[iandunn]

In 2447:

Jetpack Tweaks: Set the default Open Graph image on single posts.

Fixes #1526
Props jeherve

[iandunn]

Thanks Jeremy :)

I removed the fallback for a Jetpack Site Logo, since none of our themes support it.