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

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

This won't affect production: it will only serve static files if

- replace cc0: No Rights Reserved with pd: Public Domain
- prefix the Creative Commons License names with `cc-`
- add gfdl: GNU Free Documentation License

This should be enough for now

This reverts commit c8dd9233.