- reorder explicitily named fields,
- define `{created,updated}_{at,by}` as read-only-fields

- `HostLinkSerializer` is a `serializers.ModelSerializer` again.

- fixed image field names,
- removed fields that are no longer used

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

this renames the fields
- `ppoi` -> `image_ppoi`
- `width` -> `image_width`
- `height` -> `image_height`
and adds the fields `image_credits` and `image_alt_text`.