Fixed #27719 -- Added QuerySet.alias() to allow creating reusable aliases.

QuerySet.alias() allows creating reusable aliases for expressions that
don't need to be selected but are used for filtering, ordering, or as
a part of complex expressions.

Thanks Simon Charette for reviews.

tests/basic/tests.py

@@ -574,6 +574,7 @@ class ManagerTest(SimpleTestCase):
         'filter',
         'aggregate',
         'annotate',
+        'alias',
         'complex_filter',
         'exclude',
         'in_bulk',