Adds twitter cards and schema.org to {{ghost_head}}

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
Merge pull request #4282 from cobbspur/twitter

Adds Twitter cards to {{ghost_head}}