Add {series_sort} to the download template

It gives the series with a leading article at the end, as Calibre's default does for {series}.

tests/test_filename_template.py

@@ -55,6 +55,11 @@ def test_reference_cases_for_a_missing_series_and_for_alphabetic_order():
     assert expand("{series}", "strictly_alphabetic") == "The Saga"
 
 
+def test_series_sort_is_a_bindery_field():
+    assert expand("{series_sort}") == expand("{series_sort}", "strictly_alphabetic") == "Saga, The"
+    assert ft.expand("x{series_sort}x", ft.prepare(NO_SERIES, 42)) == "xx"
+
+
 @pytest.mark.parametrize(
     "template,expected",
     [