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/main/views/history.py

@@ -1,6 +1,5 @@
 from flask import render_template
 
-from app import current_service
 from app.main import main
 from app.utils import user_has_permissions
 
@@ -8,10 +7,4 @@ from app.utils import user_has_permissions
 @main.route("/services/<service_id>/history")
 @user_has_permissions('manage_service')
 def history(service_id):
-
-    return render_template(
-        'views/temp-history.html',
-        services=current_service.history['service_history'],
-        api_keys=current_service.history['api_key_history'],
-        events=current_service.history['events']
-    )
+    return render_template('views/temp-history.html')