Show 2 previous financial years on usage page

From a question on cross-government Slack:

> re the Usage tab - currently it shows 3 financial years - last year,
> this year and next year. is it possible to replace the "next year" tab
> with something more useful? its always going to be blank! I was
> thinking it would be good to have 2 financial years ago, 1 financial
> year ago and this financial year.

This seems like a reasonable idea, and is something we’ve talked about
before. The original intention[1] was that seeing your (unchanged) free
allowance for next year would be useful, but that doesn’t really seem to
be a user need.

***

1. See https://github.com/alphagov/notifications-admin/pull/1094
   > so that you can check what your SMS allowance is going to be before
   > you actually get into it

tests/app/main/views/organisations/test_organisation.py

@@ -499,9 +499,9 @@ def test_organisation_services_filters_by_financial_year(
     )
     mock.assert_called_once_with(ORGANISATION_ID, financial_year)
     assert normalize_spaces(page.select_one('.pill').text) == (
-        '2018 to 2019 financial year '
+        '2020 to 2021 financial year '
         '2019 to 2020 financial year '
-        '2020 to 2021 financial year'
+        '2018 to 2019 financial year'
     )
     assert normalize_spaces(page.select_one('.pill-selected-item').text) == (
         expected_selected