fix bug 843193: Demo censoring admin action that deletes uploads

* New censor() method in Submission model that sets the censored_url and
  deletes uploaded files. Tests.

* New admin action to censor selected demo Submissions, with optional
  censored_url field on confirmation page.

* Reworked admin action to delete selected demo submissions, calls
  Submission.delete() in order to ensure uploads also get deleted