Adjust OG preview images

[mukrop]

Problems (discussed with Milan and Kristi):

• OG previews get cut since both Twitter and Facebook dimensions are different from the current aspect ratio,
• Milan has to create appropriately sized versions for social networks manually.

Proposed:

• Generate appropriate OG sized images at build time (as whitespace as necessary).
• Adjust OG header to link proper Twitter and Facebook previews.

Further comments:

• In the future, we'll have "display versions" of infographics (16:9) which will be more appropriate at least for Twitter. Thus, keep in mind there will be condition generation in the future (if display version exists, it will get used instead of the normal with margins).
• Milan needs access to transformed images. For now, we'll teach him to download the appropriate image from the web. For the future, think about either generating such images into a gDrive folder or a post-commit hook on graphics repo.
• Open Graph details available here: https://ogp.me/