Allow only one review step per mentoring block.

problem_builder/public/js/mentoring_with_steps_edit.js

 function MentoringWithStepsEdit(runtime, element) {
     "use strict";
-    // Disable "add" buttons when a message of that type already exists:
-    var $buttons = $('.add-xblock-component-button[data-category=pb-message]', element);
-    var updateButtons = function() {
-        $buttons.each(function() {
-            var msg_type = $(this).data('boilerplate');
-            $(this).toggleClass('disabled', $('.xblock .submission-message.'+msg_type).length > 0);
-        });
+
+    var blockIsPresent = function(klass) {
+        return $('.xblock ' + klass).length > 0;
     };
-    updateButtons();
-    $buttons.click(function(ev) {
+
+    var updateButton = function(button, condition) {
+        button.toggleClass('disabled', condition);
+    };
+
+    var disableButton = function(ev) {
         if ($(this).is('.disabled')) {
             ev.preventDefault();
             ev.stopPropagation();
         } else {
             $(this).addClass('disabled');
         }
+    };
+
+    var initButtons = function(dataCategory) {
+        var $buttons = $('.add-xblock-component-button[data-category='+dataCategory+']', element);
+        $buttons.each(function() {
+            if (dataCategory === 'pb-message') {
+                var msg_type = $(this).data('boilerplate');
+                updateButton($(this), blockIsPresent('.submission-message.'+msg_type));
+            } else {
+                updateButton($(this), blockIsPresent('.xblock-header-sb-review-step'));
+            }
         });
+        $buttons.on('click', disableButton);
+    };
+
+    initButtons('pb-message');
+    initButtons('sb-review-step');
 
     ProblemBuilderUtil.transformClarifications(element);
     StudioEditableXBlockMixin(runtime, element);