Rerun submission after deleting a task

[ihsaan-ullah]

When you delete a task of acompetition and then rerun a submission it will get stuck in "Submitting" status because the task is not there any more

Some comments:

• do not allow deletion of tasks when they are already used
• produce error when this happens

[Didayolo]

I close this issue to keep track of it only in #1182.