Copy tree admin action.

Allow users to copy a tree of pages from the admin panel.
Refactor copy_tree and ensure all tests run

Move tests from base/tests.py to base/tests/test_models.py and
base/tests/test_utils.py

Tweak copy_tree tests for imports and refactoring
Replace FactoryBoy usage with fully isolated tests
Merge pull request #23 from jgmize/copy-tree

Copy tree