Added open graph tags for ghost head helper

issue #3900
- uses isPrivacyDisabled helper to see if useStructuredData has been disabled in config.js
- adds an array of promises to deal with asynchronous data
- resolves asynchronous data then adds open graph tags after canonical link
- featured image and tags are only added if present
- open graph tags only added on post and page
- adds unit test to check correct data is returned
- updates other unit tests to reflect changes
Merge pull request #4242 from cobbspur/meta

Adds Open Graph tags to ghost_head