Add owner to group members

The tests showed that my initial idea to loop through the queryset and call clean() does not work. So I switched to the post_init signal, which will clean the data when it is instantiated (signal needs the package blinker, which was added as a dependency). This works so well that it's hard to check the true state in the database.

There seems to be a small, flaky delay between saving changes in the database and retrieving these changes. Added a retry helper function for positive checks, but we don't want to wait for a timeout for negative checks.

Closes #2235 (closed)