Fix: broken interface of the jenkins object yielded by the setupWizard

- Callbacks registered with `onSetupWizardInitialized` are yielded a `wizard` parameter, that contained a reference to the jenkins object.
- Some plugins called the method `jenkins.initHandlebars` from this callback, to receive a Handlebars intance.
- Added a unit test to ensure this regression does not happen again

war/src/main/js/util/jenkins.js

 /**
  * Jenkins JS Modules common utility functions
  */
-import $ from 'jquery'
-import wh from 'window-handle'
+import $ from 'jquery';
+import wh from 'window-handle';
+import Handlebars from 'handlebars';
 
 var debug = false;
 var jenkins = {};
@@ -130,6 +131,13 @@ jenkins.post = function(url, data, success, options) {
 	$.ajax(args);
 };
 
+/**
+ *  handlebars setup, done for backwards compatibility because some plugins depend on it
+ */
+jenkins.initHandlebars = function() {
+	return Handlebars;
+}
+
 /**
  * Load translations for the given bundle ID, provide the message object to the handler.
  * Optional error handler as the last argument.

war/src/test/js/pluginSetupWizard.spec.js

@@ -505,4 +505,40 @@ describe("pluginSetupWizard.js", function () {
             goButton.click();
         }, { ajaxMappings, $, $body });
     });
+
+    describe('running extension callbacks', function () {
+        beforeEach(() => {
+            global.setupWizardExtensions = [];
+            global.onSetupWizardInitialized = function(extension) {
+                setupWizardExtensions.push(extension);
+            };
+
+        });
+
+        afterEach(() => {
+            delete global.setupWizardExtensions;
+            delete global.onSetupWizardInitialized;
+        });
+
+        it ('yields a jenkins object with the initHandlebars method', function(done) {
+            jsTest.onPage(function() {
+
+                $.ajax = ajaxMocks();
+
+                onSetupWizardInitialized(wizard => {
+                    const { jenkins } = wizard;
+                    expect(jenkins).toBe(getJenkins());
+
+                    // Test that the initHandlebars method returns a Handlebars instance
+                    const handlebars = jenkins.initHandlebars();
+                    expect(handlebars.registerHelper).toBeInstanceOf(Function)
+
+                    done();
+                })
+
+                const pluginSetupWizard = getSetupWizardGui();
+                pluginSetupWizard.init($body);
+            });
+        });
+    });
 });