assign_role helper function

[filipeximenes]

def remove_role(user):
    old_groups = user.groups.filter(name__in=registered_roles.keys())
    for old_group in old_groups:  # Normally there is only one, but remove all other role groups
        role = RolesManager.retrieve_role(old_group.name)
        permissions_to_remove = Permission.objects.filter(codename__in=role.permission_names_list()).all()
        user.user_permissions.remove(*permissions_to_remove)
    user.groups.remove(*old_groups)


def assign_role(user, role):
    role_cls = retrieve_role(role)

    remove_role(user)

    if role_cls:
        role_cls.assign_role_to_user(user)
        return role_cls