Fixed #27142, #27110 -- Made makemigrations consistency checks respect database routers.

Partially reverted refs #27054 except for one of the tests as this
solution supersedes that one.

Thanks Shai Berger for the review.

tests/migrations/routers.py

@@ -1,3 +1,7 @@
+class EmptyRouter(object):
+    pass
+
+
 class TestRouter(object):
     def allow_migrate(self, db, app_label, model_name=None, **hints):
         """