Added support for specifying initial values to model formsets and inline formsets.

This make them consistent with the similar capability of regular
formsets. Thanks to simon29 form the report and to Claude Paroz for the
patch.

Fixes #14574.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17373 bcc190cf-cafb-0310-a4f2-bffc1f526a37