Display if broadcast was cancelled via API

If broadcast_message has no value under cancelled_by_id, display
in the view that it was cancelled by an API call.

app/models/broadcast_message.py

@@ -181,7 +181,9 @@ class BroadcastMessage(JSONModel):
 
     @cached_property
     def cancelled_by(self):
-        return User.from_id(self.cancelled_by_id)
+        if not self.cancelled_by_id:
+            return "an API call"
+        return User.from_id(self.cancelled_by_id).name
 
     @cached_property
     def count_of_phones(self):

app/templates/views/broadcast/view-message.html

@@ -212,7 +212,7 @@
     </p>
   {% elif broadcast_message.status == 'cancelled' %}
     <p class="govuk-body">
-      Stopped by {{ broadcast_message.cancelled_by.name }}
+      Stopped by {{ broadcast_message.cancelled_by }}
       {{ broadcast_message.cancelled_at|format_datetime_human }}.
     </p>
   {% elif broadcast_message.status == 'completed' %}