Social Sharing: add metatag config for site admins to be able to alter metatags for specific nodes

[mshaver]

Overview

We don't currently expose the metatag configs to site admins on the node forms. This would be desirable for many clients as they need to customize for specific pages. There should be a dependency on #389 though, as it's a bit obscure what tokens to use so we want the correct defaults in place before handing over to clients to customize if they need to.

Dev [hrs]

• Add a "Social description" field to all content types
• Use the "Social description" field in Metatag for all description-related tags
• Add an update hook to make sure that the "Social description" field shows up in all content type form displays
• Nice to have - default to trimming the body if possible

This ticket is done when:

• I can edit any content, add a description, and that description is used for social sharing.