Add user group support

[dgets]

At the very least we're going to need to support an admin or staff level user, who should be able to see all administration data, not just his/her own. It would also be a good idea to add support for a validated group of users, and only allow them to add Substance entries, because the plebian masses really can't be trusted with such responsibility.

If the admin/staff level user isn't going to technically be one of the installation's superusers, it would be a good idea to add a user administration page, as well. Support for deleting users, changing passwords, etc, could be handled manually via this route without superuser access.

[dgets]

See also users, groups, and their permissions in django.