Django 1.7 fixes for demo site

* Use SILENCED_SYSTEM_CHECKS warning instead of test runner hack to
  avoid the warning about a change in default test runners
* Conditionally use south in the demo settings
* Add native migrations and app configs for forked apps
Use correct app name in fork_app management command

The command did not generate valid app configs. Now it does, and doesn't
raise PEP8 warnings anymore either.