Apply `futurize --stage-2` Python 2/3 compatibility transformations.

These changes specifically require the use of the "future" library.

Blogs/news only include a single OG image/video.

For now, this gives us more assurance that other sites will choose the
preview we want.

You can control the selection by adding data-ogpreview to image, video, and
source elements. data-ogpreview=0 excludes the element from being included
in the preview. Positive numbers set the preview priority. The lowest
value found is chosen first.

blog/news: Add Open Graph metadata to entry pages.

This helps other social media sites generate nice previews for these pages.