AAE-11711: add text on slack notif payload

[atchertchian]

The main text was missing from the payload: the API was generating warnings because of that:

[WARN]  web-api:WebClient:0 The top-level `text` argument is missing in the request payload for a chat.postMessage call - It's a best practice to always provide a `text` argument when posting a message. The `text` is used in places where the content cannot be rendered such as: system push notifications, assistive technology such as screen readers, etc.

I just moved existing content around to make the message more compact and closer to other github skack notifications.

Here's a sample test notification:

[erdemedeiros]

LGTM! Thank you for the improvements!