2762: Protect owner_uuid referential integrity when changing uuids and
deleting users and groups.
2762: Test assigning random non-existent owner_uuids to new objects.
2762: When deleting an object, delete permissions and fail if other
links would be left dangling.
2762: Rename assign_uuid to has_uuid to reflect expanded scope.
Merge branch 'master' into 2762-owner-uuid-integrity
2762: Move destroy_permission_links method to HasUuid.
2762: Finish enigmatic comment.
2762: Fix wrong class used in test case.
2762: Merge branch 'master' into 2762-owner-uuid-integrity
Merge branch '2762-owner-uuid-integrity'

closes #2762