Fix bug 1109121 - update custom admin actions.

This updates the censor and delete admin actions in the demos app to match the one in Django 1.4.
Refs bug #1109121 - Join path in demos app correctly.

Instead of using string substituation we should be using os.path.join.
Merge pull request #2942 from jezdez/bug1109121

Fix bug 1109121 -- fix censor admin action.