add some comments and remove a comparison that is always true from get_state_matrix
tasks can be permanently deleted in certain cases

* a user can delete a task he has assigned to another user if that user has not changed the state out of Inbox
* a user can delete a task he assigned to himself at any time

I think this also fixes a mistake I made in 7080ec9304de14912b2d54ccc072af13d32d7667 where the
new placeholder node would not be deleted if the task was the last task in a group and the
group is being hidden.