refs https://github.com/TryGhost/Team/issues/1579
- When writing to the database, the header_image is tranformed to the transformReady path
- When reading from the database, the transformReady path is transformed to an absolute path
- Includes a test when adding a newsletter
Migration:
- Updates all newsletter who have a header_image to make sure it is saved in transform ready format
- Down operation is required to work with the old model logic and transforms it back to an absolute format