Refactor summaries into model

This lets us encapsulate some of the logic that’s currently cluttering
up the view/template layer.

app/models/service.py

@@ -480,6 +480,12 @@ class Service(JSONModel):
     def has_inbound_number(self):
         return bool(self.inbound_number)
 
+    @cached_property
+    def inbound_sms_summary(self):
+        if not self.has_permission('inbound_sms'):
+            return None
+        return service_api_client.get_inbound_sms_summary(self.id)
+
     @cached_property
     def all_template_folders(self):
         return sorted(
@@ -660,3 +666,7 @@ class Service(JSONModel):
         ):
             if test:
                 yield BASE + '_incomplete' + tag
+
+    @cached_property
+    def returned_letter_summary(self):
+        return service_api_client.get_returned_letter_summary(self.id)