slack_incoming: add ok=true to json in case of success.

This better simulates the Slack API, which is important, since some
integrations check this response and decide whether the Slack endpoint
is working based on what they receive.

zerver/webhooks/slack_incoming/view.py

@@ -78,7 +78,7 @@ def api_slack_incoming_webhook(
     if body != "":
         body = replace_formatting(replace_links(body).strip())
         check_send_webhook_message(request, user_profile, user_specified_topic, body)
-    return json_success(request)
+    return json_success(request, data={"ok": True})
 
 
 def render_block(block: WildValue) -> str: