app_index: use add_url instead of admin_url+"/add"

admin_url is not defined if the user has no change permissions, but is
allowed to add objects.

Django 1.6.5.

From Django's `contrib/admin/sites.py`:

    if perms.get('change', False):
        try:
            model_dict['admin_url'] = reverse('admin:%s_%s_changelist' % info, current_app=self.name)
        except NoReverseMatch:
            pass
    if perms.get('add', False):
        try:
            model_dict['add_url'] = reverse('admin:%s_%s_add' % info, current_app=self.name)
        except NoReverseMatch:
            pass