After reading this blogpost by Thoughtbot I got the idea to implement a simple bookmarklet to help me create these beautiful PR messages in slack.
If you want it, too, simply create a bookmark with the following Javascript as link:
javascript: (function() {const diffStat = document.getElementById("diffstat").textContent.replace(/(\r\n\t|\n|\r\t)/gm,"").trim().replace(/\s\s+/g, ' '); window.prompt("Copy this", ":pullrequest: " + window.location.toString() + " " + diffStat);})();
Navigate to a GitHub PR and click on the bookmark. You will see a messagebox where you can copy the text and paste it into your slack. Sadly Firefox permissions won't allow us to copy the text directly into the clipboard.