[1.8.x] Fixed #24351, #24346 -- Changed the signature of allow_migrate().

The new signature enables better support for routing RunPython and
RunSQL operations, especially w.r.t. reusable and third-party apps.

This commit also takes advantage of the deprecation cycle for the old
signature to remove the backward incompatibility introduced in #22583;
RunPython and RunSQL won't call allow_migrate() when when the router
has the old signature.

Thanks Aymeric Augustin and Tim Graham for helping shape up the patch.

Refs 22583.

Conflicts:
	django/db/utils.py

Backport of bed504d70b from master

django/db/backends/base/creation.py

@@ -404,7 +404,7 @@ class BaseDatabaseCreation(object):
         def get_objects():
             for model in serializers.sort_dependencies(app_list):
                 if (not model._meta.proxy and model._meta.managed and
-                        router.allow_migrate(self.connection.alias, model)):
+                        router.allow_migrate_model(self.connection.alias, model)):
                     queryset = model._default_manager.using(self.connection.alias).order_by(model._meta.pk.name)
                     for obj in queryset.iterator():
                         yield obj