Fix: Make sure clicking buttons causes corresponding child to be added

after deleting child that can only be added once.

This is a follow-up to the previous commit. It is now possible to add
and remove children ad infinitum, irrespective of whether the parent
block allows single or multiple copies of them.

problem_builder/public/js/mentoring_with_steps_edit.js

 function MentoringWithStepsEdit(runtime, element) {
     "use strict";
 
+    var $buttons = $('.add-xblock-component-button[data-category=sb-review-step]', element);
+
     var blockIsPresent = function(klass) {
         return $('.xblock ' + klass).length > 0;
     };
@@ -18,21 +20,26 @@ function MentoringWithStepsEdit(runtime, element) {
         }
     };
 
-    var initButtons = function(dataCategory) {
-        var $buttons = $('.add-xblock-component-button[data-category='+dataCategory+']', element);
-        $buttons.each(function() {
-            updateButton($(this), blockIsPresent('.xblock-header-sb-review-step'));
+    var updateButtons = function(buttons) {
+        buttons.each(function() {
+            var button = $(this);
+            updateButton(button, blockIsPresent('.xblock-header-sb-review-step'));
         });
+    };
+
+    var initButtons = function() {
+        updateButtons($buttons);
         $buttons.on('click', disableButton);
     };
 
-    var updateButtons = function() {
-        initButtons('sb-review-step');
+    var resetButtons = function() {
+        var $disabledButtons = $buttons.filter('.disabled');
+        updateButtons($disabledButtons);
     };
 
     ProblemBuilderUtil.transformClarifications(element);
 
-    updateButtons();
-    runtime.listenTo('deleted-child', updateButtons);
+    initButtons();
+    runtime.listenTo('deleted-child', resetButtons);
 
 }

problem_builder/public/js/review_step_edit.js

 function ReviewStepEdit(runtime, element) {
     "use strict";
 
+    var $buttons = $('.add-xblock-component-button[data-category=pb-message]', element);
+
     var blockIsPresent = function(klass) {
         return $('.xblock ' + klass).length > 0;
     };
@@ -18,22 +20,27 @@ function ReviewStepEdit(runtime, element) {
         }
     };
 
-    var initButtons = function(dataCategory) {
-        var $buttons = $('.add-xblock-component-button[data-category='+dataCategory+']', element);
-        $buttons.each(function() {
-            var msg_type = $(this).data('boilerplate');
-            updateButton($(this), blockIsPresent('.submission-message.'+msg_type));
+    var updateButtons = function(buttons) {
+        buttons.each(function() {
+            var button = $(this);
+            var msgType = button.data('boilerplate');
+            updateButton(button, blockIsPresent('.submission-message.'+msgType));
         });
+    };
+
+    var initButtons = function() {
+        updateButtons($buttons);
         $buttons.on('click', disableButton);
     };
 
-    var updateButtons = function() {
-        initButtons('pb-message');
+    var resetButtons = function() {
+        var $disabledButtons = $buttons.filter('.disabled');
+        updateButtons($disabledButtons);
     };
 
     ProblemBuilderUtil.transformClarifications(element);
 
-    updateButtons();
-    runtime.listenTo('deleted-child', updateButtons);
+    initButtons();
+    runtime.listenTo('deleted-child', resetButtons);
 
 }