Set reaction's z-index to show the tooltip in front of other comments (#20035)

app/assets/javascripts/initializers/initializeCommentsPage.js.erb

@@ -95,6 +95,7 @@ function initializeCommentsPage() {
     var butts = document.getElementsByClassName('reaction-button');
     for (var i = 0; i < butts.length; i++) {
       var butt = butts[i];
+      butt.style.zIndex = butts.length - i;
       butt.onclick = function (event) {
         var thisButt = this;
         event.preventDefault();