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/set-inbound-sms.html

@@ -13,7 +13,7 @@
       <h1 class="heading-large">Receive text messages</h1>
       {% if 'inbound_sms' in current_service.permissions %}
         <p>
-          Your service can receive text messages sent to {{ inbound_number }}.
+          Your service can receive text messages sent to {{ current_service.inbound_number }}.
         </p>
         <p>
           If you want to turn this feature off,