This reverts commit 18dd866f.

Make Category.subtitle a CharField

- Replace ImageField with ForeignKey reference to Image
- Update `program/show.json` fixture
- Update serializer to refer to Image and allow null

Konrad Mohrfeldt authored 2 years ago and Ernesto Rico Schmidt committed 2 years ago

The image attributes on our models were using VersatileImageField before
our migration to a separate image model. VersatileImageField uses a
VARCHAR field internally that cannot be converted to a ForeignKey.

Signed-off-by: Ernesto Rico Schmidt <ernesto@helsinki.at>