[Core] Images refactor
[Core] Images refactor

The `code` field name has been changed to `type`, and is no longer
unique; in addition it became editable upon editing by UI.
Product/Taxon `getImageByCode` has been refactored to `getImagesByType`,
now returning a collection, instead of a single image.

Sylius/SyliusUiBundle,Sylius/SyliusUiBundle