[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/sites_tests/tests.py

@@ -137,7 +137,7 @@ class SitesFrameworkTests(TestCase):
 
 
 class JustOtherRouter(object):
-    def allow_migrate(self, db, model):
+    def allow_migrate(self, db, app_label, **hints):
         return db == 'other'