made the check_copy_relations check a bit smarter by taking superclasses in account

fixes #1723

There's one test failure, but that happens on develop as well

    ======================================================================
    FAIL: test_page_language_url (cms.tests.no_i18n.TestNoI18N)
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    Traceback (most recent call last):
      File "/home/benjamin/projects/django-cms/cms/tests/no_i18n.py", line 67, in test_page_language_url
        self.assertEqual(url, "%s" % path)
    AssertionError: u'/en/' != '/'

    ----------------------------------------------------------------------
    Ran 503 tests in 106.024s