closes#3900
- Adds twitter cards to ghost head helper
- Adds schema json information
- Adds test with null values for post image and cover image
- Adds test for privacy flag
- Adds test for the case of no tags
- Updates test to check for twitter card and schema data
- Updates privacy.md
- Fixes issue with image urls that are linked by url rather than uploaded
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
