Refactor creator/owner/modifier code (#592).

The internal Access Control code reflects the DM 4.5 access control concept better:
- The "owner" applies only to workspaces. Regular topics/associations does not have an owner.
- The topic/association's "creator" and "modifier" are purely cosmetic. The creator does not govern the access control rules anymore. Creator and owner are maintained for informational purpose only.
- As a consequence the creator must not be faked anymore. That means it is not necessary anymore to set "admin" as the creator of things which are actually created by the system itself. These things have no creator anymore. It is just undefined.

See #592.