Currently tasks are always assigned to the current user.

In some circumstances, it would be better to be able to leave a task unassigned. This way it would show up on everyone's todo list and scheduler views.

Assigning to 'admin' just because there's no other known owner for a task is a bit hacky. Specifying no-ownership for such tasks would be cleaner.

Done in r129. Todos can now be assigned to "null user" by selecting the empty option from owner selection box.

When a task is assigned to null user, it can be viewed by all users and it will end up in everyone's todo list.