Factor out code that gets message content

The nesting is getting pretty deep here. Let’s make it into its own
method so it doesn’t get out of hand when we add more functionality to
it.

app/main/views/conversation.py

@@ -120,12 +120,7 @@ def get_sms_thread(service_id, user_number):
         yield {
             'inbound': is_inbound,
             'content': SMSPreviewTemplate(
-                {
-                    'content': (
-                        notification['content'] if is_inbound else
-                        notification['template']['content']
-                    )
-                },
+                {'content': get_sms_content(notification, is_inbound)},
                 notification.get('personalisation'),
                 downgrade_non_gsm_characters=(not is_inbound),
                 redact_missing_personalisation=redact_personalisation,
@@ -134,3 +129,10 @@ def get_sms_thread(service_id, user_number):
             'status': notification.get('status'),
             'id': notification['id'],
         }
+
+
+def get_sms_content(notification, is_inbound):
+    return (
+        notification['content'] if is_inbound else
+        notification['template']['content']
+    )