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/models/service.py

@@ -1,3 +1,5 @@
+from operator import attrgetter
+
 from flask import Markup, abort, current_app
 from notifications_utils.field import Field
 from notifications_utils.formatters import nl2br
@@ -5,6 +7,7 @@ from notifications_utils.take import Take
 from werkzeug.utils import cached_property
 
 from app.models import JSONModel
+from app.models.event import APIKeyEvents, ServiceEvents
 from app.models.organisation import Organisation
 from app.models.user import InvitedUsers, User, Users
 from app.notify_client.api_key_api_client import api_key_api_client
@@ -632,6 +635,9 @@ class Service(JSONModel):
             if test:
                 yield BASE + '_incomplete' + tag
 
-    @cached_property
+    @property
     def history(self):
-        return service_api_client.get_service_history(self.id)['data']
+        return sorted(
+            ServiceEvents(self.id) + APIKeyEvents(self.id),
+            key=attrgetter('time'),
+        )