FIX: Quoting a quote preserves the original post information (#8746)

Let's say post #2 quotes post number #1. If a user decides to quote the
quote in post #2, it should keep the information of post #1
("user_1, post: 1, topic: X"), instead of replacing with current post
info ("user_2, post: 2, topic: X").

app/assets/javascripts/pretty-text/engines/discourse-markdown/quotes.js.es6

@@ -73,6 +73,10 @@ const rule = {
       token.attrs.push(["class", "quote no-group"]);
     }
 
+    if (username) {
+      token.attrs.push(["data-username", username]);
+    }
+
     if (postNumber) {
       token.attrs.push(["data-post", postNumber]);
     }