Fix calculating remaining free allowance for SMS

The way it was done before, the remainder was incorrect in the
billing report and in the org usage query - it was the sms remainder
left at the start of the report period, not at the end of that period.

This became apparent when we tried to show sms_remainder on the org
usage report, where start date is always the start of the financial year.
We saw that sms sent by services did not reduce their free allowance
remainder according to the report. As a result of this, we had to
temporarily remove of sms_remainder column from the report, until
we fix the bug - it has been fixed now, yay!

I think the bug has snuck in partially because our fixtures for testing
this part of the code are quite complex, so it was
harder to see that numbers don't add up. I have added comments
to the tests to try and make it a bit clearer why the results are
as they are.

I also added comments to the code, and renamed some variables,
to make it easier to understand, as there are quite a few
moving parts in it - subqueries and the like.
I also renamed the fetch_sms_free_allowance_remainder method to
fetch_sms_free_allowance_remainder_until_date so it is clearer
what it does.

tests/app/organisation/test_rest.py

@@ -831,7 +831,7 @@ def test_get_organisation_services_usage(admin_request, notify_db_session):
     assert service_usage['free_sms_limit'] == 10
     assert service_usage['letter_cost'] == 0
     assert service_usage['sms_billable_units'] == 19
-    assert service_usage['sms_remainder'] == 10
+    assert service_usage['sms_remainder'] == 0
     assert service_usage['sms_cost'] == 0.54