This changes the owner when anyone edits it. If an admin edits a TaskArea I own, suddenly I can't manage it anymore because the owner changed.

Perhaps make this `creator` instead of `owner` and only set it when a new TaskArea is created? That way you can still restrict deletion and stuff to creators but allow admins to make changes as well.

As for de-duplicating this code, a mixin is the solution (inheriting from a base class wouldn't work because the ModelAdmins have different base classes, but a mixin can override `save_model` in both.