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.

tests/gis_tests/layermap/tests.py

@@ -322,7 +322,7 @@ class OtherRouter(object):
     def allow_relation(self, obj1, obj2, **hints):
         return None
 
-    def allow_migrate(self, db, model):
+    def allow_migrate(self, db, app_label, **hints):
         return True