Don't need to call ORM::load_relationships() in activate() because it's already called in _add_to_path()
Move the relevant phpDoc from initialize() to load_relationships().  Fix the default value of _relationship_cache in reinitialize().
It looks like this was not necessary.

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Revert "Implement IdentityProvider_GroupDefinition - we expect that in Access."

This reverts commit 99899d13361248a02d32a2375ba57072e1239c16.
Update validation rules for Model_Item to the new K3 format.