Make a service model and use for permissions

Having the service floating about as JSON is a bit flakey. Could easily
introduce a mistake where you mistype the name of a key and silently
get `None`.

Also means doing awkward things like `if 'permission' in
current_service['permissions']`, whereas for users we can do the
much cleaner `user.has_permission()`.

So this commit:
- introduces a model
- adds a `.has_permission` method similar to the one we have for users

app/main/views/dashboard.py

@@ -152,7 +152,7 @@ def usage(service_id):
     yearly_usage = billing_api_client.get_service_usage_ft(service_id, year)
 
     usage_template = 'views/usage.html'
-    if 'letter' in current_service['permissions']:
+    if current_service.has_permission('letter'):
         usage_template = 'views/usage-with-letters.html'
     return render_template(
         usage_template,
@@ -293,7 +293,7 @@ def get_dashboard_partials(service_id):
     stats = service_api_client.get_service_statistics(service_id, today_only=False)
     column_width, max_notifiction_count = get_column_properties(
         number_of_columns=(
-            3 if 'letter' in current_service['permissions'] else 2
+            3 if current_service.has_permission('letter') else 2
         )
     )
     dashboard_totals = get_dashboard_totals(stats),
@@ -313,7 +313,7 @@ def get_dashboard_partials(service_id):
             'views/dashboard/_inbox.html',
             inbound_sms_summary=(
                 service_api_client.get_inbound_sms_summary(service_id)
-                if 'inbound_sms' in current_service['permissions'] else None
+                if current_service.has_permission('inbound_sms') else None
             ),
         ),
         'totals': render_template(