Enable and customize open graph tags

[ryelle]

This PR force-enables the Jetpack Open Graph behavior, and then runs over the values to customize them slightly. Mostly to make sure they're consistent and accurate for each value/page.

I've used the main developer.w.org's default image, but we can easily swap that out before merging this PR if there's a DevBlog specific one.

Fixes #53

Screenshots

I've been testing by manually checking the generated code, so that's what I'm sharing here.

Homepage: uses the default image, site title, and site description.

Article, with a featured image: uses the featured image, post title, and excerpt.

Article without a featured image: uses the default image, post title, and excerpt.

Category (& other archive pages): uses the default image, archive title, and description (omitted if not available or relevant).

To test

• Load any page, view source or inspect the head
• Find <!-- Jetpack Open Graph Tags -->
• The tags should be accurate to the page they are on

(I know there's a way to test locally with a tunnel, but I haven't tried that.)

[justintadlock]

I was able to review the code, which looks good to me. I don't have a full setup to test the site right now. @ndiego might want to do a final sign-off.

[ndiego]

I was able to review the code, which looks good to me. I don't have a full setup to test the site right now. @ndiego might want to do a final sign-off.

You both know more about Open Graph tag implementation than I do 😅, but this looks good to me with the new image in #53

[adamwoodnz]

LGTM too