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-number.html

@@ -10,7 +10,7 @@
 
 {% block maincolumn_content %}
     <h1 class="heading-large">Set Inbound Number</h1>
-    {% if service_has_inbound_number %}
+    {% if current_service.has_inbound_number %}
     <p> This service already has an inbound number </p>
     {{ page_footer(
 	    back_link=url_for('.service_settings', service_id=current_service.id),