[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/migrations/test_multidb.py

@@ -16,7 +16,7 @@ class AgnosticRouter(object):
     """
     A router that doesn't have an opinion regarding migrating.
     """
-    def allow_migrate(self, db, model, **hints):
+    def allow_migrate(self, db, app_label, **hints):
         return None
 
 
@@ -24,7 +24,7 @@ class MigrateNothingRouter(object):
     """
     A router that doesn't allow migrating.
     """
-    def allow_migrate(self, db, model, **hints):
+    def allow_migrate(self, db, app_label, **hints):
         return False
 
 
@@ -32,7 +32,7 @@ class MigrateEverythingRouter(object):
     """
     A router that always allows migrating.
     """
-    def allow_migrate(self, db, model, **hints):
+    def allow_migrate(self, db, app_label, **hints):
         return True
 
 
@@ -40,7 +40,7 @@ class MigrateWhenFooRouter(object):
     """
     A router that allows migrating depending on a hint.
     """
-    def allow_migrate(self, db, model, **hints):
+    def allow_migrate(self, db, app_label, **hints):
         return hints.get('foo', False)