Open Graph Protocol (OGP) meta tags

Adds Open Graph Protocol (OGP) meta tags to some CiviCRM pages, which provides a better experience when sharing pages on social media.

Usage:

• Event Registration/Event Info Page
  • The event 'summary' field (on the first event settings page) is used for the og:description tag.
  • The first image found in the event 'intro text' (from the online registration settings) is used for the og:image tag.
  • If no image is found, the site logo (from ogp settings) is used for the og:image tag.
• Contribution pages
  • The site logo is used as the og:image (this might change in the future, if an image is found in the intro text).
• Profiles
  • If the first field of a profile is a markup/html field, and it includes an image, that image will be used.
  • If no image found, fallsback on the site logo.
  • If a profile description is found, it is used as the og:description (note: this could change in the future, since the description is usually meant to be administrative).