Usage for all services is a platform admin report that groups letters by
postage. We want it to show `europe` and `rest-of-world` letters under a
single category of `international`, so this updates the query to do
that and to order appropriately.
bst_date is a date field. Comparing dates with datetimes in postgres
gets confusing and dangerous. See this example, where a date evaluates
as older than midnight that same day.
```
notification_api=# select '2019-04-01' >= '2019-04-01 00:00';
?column?
----------
f
(1 row)
```
By only using dates everywhere, we reduce the chance of these bugs
happening
Created a platform-stats blueprint and moved the new platform stats
endpoint to the new blueprint (it was previously in the service
blueprint). Since the original platform stats route and the new platform
stats route are now in different blueprints, their view functions can
have the same name without any issues.
