[637/#1038] Org hierarchy methods in CKAN core preferred to publisher.py

lib/publisher.py generally pruned since ckan/model/group.py now has much
of the functionality.

Cope with get_children_groups now returning group objects rather than
dicts.

Tests tidied - create_test_data now does hierarchy and sysadmin rights
properly, so tests pass: tests/lib/test_publisher.py
Merge branch '2.0' of github.com:datagovuk/ckanext-dgu into 2.0