Adds Twitter cards to {{ghost_head}}

Issue #3900

- Adds twitter cards to ghost head helper
- Updates test to check for twitter card data
- Updates privacy.md
- Fixes issue with image urls that are linked by url rather than uploaded
Yeah, per my comment below, it'd be cool to just have all of this "seo meta stuff" inside of one object that you then iterate over in one loop to build out all your meta tags