Munge stuff into a consistent event data type

We store our audit history in two ways:

  1. A list of versions of a service
  2. A list of events to do with API keys

In the future there could be auditing data which we want to display that
is stored in other formats (for example the event table).

This commit adds some objects which wrap around the different types of
auditing data, and expose a consistent interface to them. This
architecture will let us:
- write clean code in the presentation layer to display these events on
  a page
- add more types of events in the future by subclassing the `Event` data
  type, without having to rewrite anything in the presentation layer

app/__init__.py

@@ -3,7 +3,6 @@ import os
 import urllib
 from datetime import datetime, timedelta, timezone
 from functools import partial
-from numbers import Number
 from time import monotonic
 
 import ago
@@ -84,7 +83,7 @@ from app.notify_client.template_statistics_api_client import (
     template_statistics_client,
 )
 from app.notify_client.user_api_client import user_api_client
-from app.utils import get_logo_cdn_domain, id_safe
+from app.utils import format_thousands, get_logo_cdn_domain, id_safe
 
 login_manager = LoginManager()
 csrf = CSRFProtect()
@@ -351,14 +350,6 @@ def format_delta(date):
     )
 
 
-def format_thousands(value):
-    if isinstance(value, Number):
-        return '{:,.0f}'.format(value)
-    if value is None:
-        return ''
-    return value
-
-
 def valid_phone_number(phone_number):
     try:
         validate_phone_number(phone_number)