Fixed #7758 and #17189 -- Allowed to override the `form_url` context var in the admin change view and the user admin's password change view. Thanks, michal and krzysztof.szczesny.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17466 bcc190cf-cafb-0310-a4f2-bffc1f526a37