- remove ThumbnailsMixin and thumbnails fields,
- remove image_* fields and theis usage,
- use utils.delete_links function

- remove ThumbnailsMixin and thumbnails fields,
- remove image_* fields and theis usage,
- use utils.delete_links function

- remove ThumbnailsMixin and thumbnails fields,
- remove image_* fields and theis usage,
- introduce utils.delete_links function

Use it with `HostSerializer`, `NoteSerializer` and `ShowSerializer`

- 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

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

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