[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

docs/internals/deprecation.txt

@@ -168,6 +168,10 @@ details on these changes.
 * Ability to specify ``ContentType.name`` when creating a content type instance
   will be removed.
 
+* Support for the old signature of ``allow_migrate`` will be removed. It changed
+  from ``allow_migrate(self, db, model)`` to
+  ``allow_migrate(self, db, app_label, model_name=None, **hints)``.
+
 .. _deprecation-removed-in-1.9:
 
 1.9