Social Previews: Improve the heuristic for determining preview content

Calypso SEO preview uses several more advanced ways to gather possible preview content from the post. With the port to Jetpack under the name Social Previews, we have started with the basic implementation - using title, perex, post content and a featured image.

We should see how we can improve this. To be sure, the previews are as close as possible to the real outcome, we need to check how does WP with our plugins generate meta tags, which are used as a source for the previews in external services.

It's worth noting these meta tags could be generated and altered by different plugins (I imagine Jetpack and Yoast could both touch this).

I imagine a few ways to do this:

- We can see what Jetpack does under the hood to generate meta tags (or is it just pure WP feature?) and match it perfectly, since our previews live in Jetpack too.
- We can make this more compatible and see if there is a way to load the render the post on the server and read finalized meta tags ourselves. This will add a loading step but will make sure any plugin combo will be respected and our previews more accurate.

No files, tests, or entry points are identified. Start by investigating how Jetpack and WordPress generate and alter social meta tags, including interactions with plugins such as Yoast, and compare that with the current Social Previews inputs. Done requires a defined approach and validation that previews match the finalized metadata.

Written by the indexing model from the issue text.