Any frontend will need the dimensions of the supplied thumbnails
to select the appropriate thumbnail in different rendering contexts.

Apart from that the thumbnail name is not that helpful, as we need
the full thumbnail URL to be able to display it.

- 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