Fixed #13407 -- Corrected verbose names for autogenerated m2m models, and...

Fixed #13407 -- Corrected verbose names for autogenerated m2m models, and cleaned up the default form prefix when an autogenerated m2m through model is used in a formset. Thanks to carljm for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13029 bcc190cf-cafb-0310-a4f2-bffc1f526a37