[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

tests/cache/tests.py

@@ -959,14 +959,17 @@ class DBCacheRouter(object):
     def db_for_read(self, model, **hints):
         if model._meta.app_label == 'django_cache':
             return 'other'
+        return None
 
     def db_for_write(self, model, **hints):
         if model._meta.app_label == 'django_cache':
             return 'other'
+        return None
 
-    def allow_migrate(self, db, model):
-        if model._meta.app_label == 'django_cache':
+    def allow_migrate(self, db, app_label, **hints):
+        if app_label == 'django_cache':
             return db == 'other'
+        return None
 
 
 @override_settings(