Extract formatters into their own module

We have lots of functions for converting various types of data into
strings to be displayed to the user somewhere.

This commit collects all these functions into their own module, rather
than having them cluttering up `app/__init__.py` or buried amongst
various other things that have ended up in `app/utils.py`.

app/main/views/dashboard.py

@@ -20,11 +20,10 @@ from werkzeug.utils import redirect
 from app import (
     billing_api_client,
     current_service,
-    format_date_numeric,
-    format_datetime_numeric,
     service_api_client,
     template_statistics_client,
 )
+from app.formatters import format_date_numeric, format_datetime_numeric
 from app.main import main
 from app.statistics_utils import get_formatted_percentage
 from app.utils import (