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/models/event.py

@@ -2,9 +2,9 @@ from abc import ABC, abstractmethod
 
 from notifications_utils.formatters import formatted_list
 
+from app.formatters import format_thousands
 from app.models import ModelList
 from app.notify_client.service_api_client import service_api_client
-from app.utils import format_thousands
 
 
 class Event(ABC):