Grappelli container class

[jpotterm]

We discussed this previously in https://github.com/django-polymorphic/django-polymorphic-tree/pull/48 but it looks like nothing was ever done. Now the current release of Grappelli is incompatible with this package's nodetree_grappelli.css. It needs to use either .grp-container or #container but not #grp-container.