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/queries/test_qs_combinators.py

@@ -303,6 +303,7 @@ class QuerySetSetOperationTests(TestCase):
             combinators.append('intersection')
         for combinator in combinators:
             for operation in (
+                'alias',
                 'annotate',
                 'defer',
                 'delete',