Fix page copy and replace_text to pick up child relations that are defined on a superclass

Further to https://github.com/torchbox/django-modelcluster/issues/9 - there are a few places in Wagtail that are inspecting the `child_relations` meta property themselves, namely the Page.copy() method and the replace_text management command. We now know that this is wrong, since `child_relations` doesn't include relations defined on a superclass of the model. This PR adds tests for the failure cases, and fixes them by using modelcluster's new `get_all_child_relations` helper function.