Tasks can be assigned to users. An user may decide to "claim" a task
directly, which will assign the task to him. Otherwise, it is possible
to set some arbitrary user as the assignee or remove the assignee
completely through the edition form.
Marking a task as completed will remove the assignee, while
re-activating a task will assign it to the user who re-activated it.
Also, fixed a bug which allowed a completed task to be edited.
