Add OrderHistory class

- the main purpose of this class is to manage querying the order totals
  from the database and slice various kinds of time series out of the
  data
- the class holds the former `aggregate_orders()` function as a method
- modularize the corresponding tests
- add `tests.config` with globals used when testing to provide a
  single source of truth for various settings

tests/forecasts/timify/__init__.py

@@ -0,0 +1 @@
+"""Tests for the `urban_meal_delivery.forecasts.timify` module."""