jira: Fix handling of sprint_started events.

Now that we inspect more attributes of events in determining subjects,
we need to filter events like sprint_started before fetching the subject.

zerver/webhooks/jira/view.py

@@ -316,11 +316,11 @@ def api_jira_webhook(request: HttpRequest, user_profile: UserProfile,
                      payload: Dict[str, Any]=REQ(argument_type='body')) -> HttpResponse:
 
     event = get_event_type(payload)
-    subject = get_issue_subject(payload)
-
     if event in IGNORED_EVENTS:
         return json_success()
 
+    subject = get_issue_subject(payload)
+
     content_func = get_event_handler(event)
 
     if content_func is None: