Refactor seo.html and support OpenGraph image overrides (#1321)

- Refactor seo.html include and add `page.header.og_image` override
- DRY up handling of page images (`page.header.image`, `page.header.overlay_image`, `page.header.teaser`, and `site.og_image`).
- Allow page images to be set with `header.og_image` page variable in YAML Front Matter as per feature request in #1316.
- Add sample posts for testing `og_image` override
- Set author bio as site OpenGraph default image
- Document `page.header.og_image` OpenGraph override
- Update CHANGELOG and history

Close #1316

test/_config.yml

@@ -63,7 +63,7 @@ facebook:
   username               :
   app_id                 :
   publisher              :
-og_image                 : # Open Graph/Twitter default site image
+og_image                 : "/assets/images/bio-photo.jpg"
 # For specifying social profiles
 # - https://developers.google.com/structured-data/customize/social-profiles
 social: