Refactor settings page to use service model

There’s a lot of code in service settings which:
- talks to the API directly through the clients
- passes that information through to the Jinja template

By encapsulating this logic in the service model:
- the Jinja template can access the data directly
- the logic can be reused across multiple methods

app/templates/views/service-settings/data-retention.html

@@ -25,7 +25,7 @@
       </div>
   </div>
   <div class="user-list">
-    {% if not data_retention_settings %}
+    {% if not current_service.data_retention %}
       <div class="user-list-item">
         <span class="hint">You haven’t added any data retention</span>
       </div>
@@ -36,7 +36,7 @@
         field_headings_visible=False,
         caption_visible=False
       ) %}
-        {% for item in data_retention_settings %}
+        {% for item in current_service.data_retention %}
             {% call row() %}
               {{ text_field(item.notification_type | format_notification_type)}}
               {{ text_field(item.days_of_retention) }}