entity: fix SAWarning: declare "owner"" and "creator"" relations only for direct
subclasses. Otherwise when using subclasses of direct subclasses this the warning:

sqlalchemy/orm/properties.py:909: SAWarning: Warning: relationship 'creator' on
mapper 'Mapper|Subsubclass|table' supersedes the same relationship on inherited
mapper 'Mapper|Subclass|table'; this can cause dependency issues during flush

This would happen for subclasses with polymorphism by single table inheritance
for example