Stopped staticfiles app from requiring a models module when looking for static...

Stopped staticfiles app from requiring a models module when looking for static files. Also removed a bit of code smell in the prefix handling by saving it in the source file storage.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15219 bcc190cf-cafb-0310-a4f2-bffc1f526a37