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/utils.py

@@ -6,6 +6,7 @@ from datetime import datetime, time, timedelta, timezone
 from functools import wraps
 from io import BytesIO, StringIO
 from itertools import chain
+from numbers import Number
 from os import path
 from urllib.parse import urlparse
 
@@ -602,3 +603,11 @@ class PermanentRedirect(RequestRedirect):
     and Windows 8.1, so this class keeps the original status code of 301.
     """
     code = 301
+
+
+def format_thousands(value):
+    if isinstance(value, Number):
+        return '{:,.0f}'.format(value)
+    if value is None:
+        return ''
+    return value