closes https://github.com/TryGhost/Ghost/issues/9031
- add a default value `unsplash` value to the `setting` model so that Unsplash is activated when the server doesn't return an `unsplash` setting
- update the `unsplash-settings` transform to always deserialize or serialize to `{isActive: true}` when the value is blank or not parsable
- add acceptance regression test covering API not returning an `unplash` setting
- add unit tests for the `unsplash-settings` transform
6a9239974f