- Because we might now get images from the content item or locally-supplied, we check the url is a url - if it is, we return it, if not, we try to make an image_path for it and return that.

- this will replace individual image handling in hero and featured blocks.
  Tests to be added in the commits for the consuming block types.
- to simplify calls, we load the sources into an OpenStruct. This is
  close to how they're currently handled in the blocks, but removes the
  Dataclass strictness (which allows this to be used more generally).

- Update tests and fixtures (local files to be updated in a later commit)