experimental flag to run Jenkins without YUI

The YUI library is old and no longer maintained.

Add a user experimental flag to disable YUI. It's disabled by default.
When enabling all the YUI related js libraries and css classes are not
loaded.

Following PR are required to get Jenkins to not show any errors

eventually jenkinsci#7569

Some plugins that use YUI (not complete):
credentials
ldap
global-build-stats
build-monitor
categorized-view

Plugins that make use of makeButton (not complete)
credentials (fixed with jenkinsci/credentials-plugin#533)
openid

acceptance-test-harness

core/src/main/java/jenkins/model/experimentalflags/RemoveYuiUserExperimentalFlag.java

@@ -0,0 +1,22 @@
+package jenkins.model.experimentalflags;
+
+import edu.umd.cs.findbugs.annotations.Nullable;
+import hudson.Extension;
+
+@Extension
+public class RemoveYuiUserExperimentalFlag extends BooleanUserExperimentalFlag {
+    public RemoveYuiUserExperimentalFlag() {
+        super("remove-vui.flag");
+    }
+
+    @Override
+    public String getDisplayName() {
+        return "Remove YUI";
+    }
+
+    @Nullable
+    @Override
+    public String getShortDescription() {
+        return "Remove YUI from all Jenkins UI pages. This will break anything that depends on YUI";
+    }
+}

core/src/main/resources/lib/layout/layout.jelly

@@ -128,31 +128,36 @@ THE SOFTWARE.
 
     <st:adjunct includes="org.kohsuke.stapler.bind"/>
 
-    <!-- To use the debug version of YUI, set the system property 'debug.YUI' to true -->
-    <j:set var="yuiSuffix" value="${h.yuiSuffix}" />
-    <l:yui module="yahoo" />
-    <l:yui module="dom" />
-    <l:yui module="event" />
-    <j:if test="${h.yuiSuffix=='debug'}">
-      <l:yui module="logger" />
+    <l:userExperimentalFlag var="removeYUI" flagClassName="jenkins.model.experimentalflags.RemoveYuiUserExperimentalFlag" />
+    <j:if test="${!removeYUI}">
+      <!-- To use the debug version of YUI, set the system property 'debug.YUI' to true -->
+      <j:set var="yuiSuffix" value="${h.yuiSuffix}" />
+      <l:yui module="yahoo" />
+      <l:yui module="dom" />
+      <l:yui module="event" />
+      <j:if test="${h.yuiSuffix=='debug'}">
+        <l:yui module="logger" />
+      </j:if>
+      <l:yui module="animation" />
+      <l:yui module="dragdrop" />
+      <l:yui module="container" />
+      <l:yui module="connection" />
+      <l:yui module="datasource" />
+      <l:yui module="autocomplete" />
+      <l:yui module="menu" />
+      <l:yui module="element" />
+      <l:yui module="button" />
+      <l:yui module="storage" />
     </j:if>
-    <l:yui module="animation" />
-    <l:yui module="dragdrop" />
-    <l:yui module="container" />
-    <l:yui module="connection" />
-    <l:yui module="datasource" />
-    <l:yui module="autocomplete" />
-    <l:yui module="menu" />
-    <l:yui module="element" />
-    <l:yui module="button" />
-    <l:yui module="storage" />
 
     <script src="${resURL}/scripts/hudson-behavior.js" type="text/javascript"></script>
     <script src="${resURL}/scripts/sortable.js" type="text/javascript"/>
 
-    <link rel="stylesheet" href="${resURL}/scripts/yui/container/assets/container.css" type="text/css"/>
-    <link rel="stylesheet" href="${resURL}/scripts/yui/container/assets/skins/sam/container.css" type="text/css"/>
-    <link rel="stylesheet" href="${resURL}/scripts/yui/menu/assets/skins/sam/menu.css" type="text/css" />
+    <j:if test="${!removeYUI}">
+      <link rel="stylesheet" href="${resURL}/scripts/yui/container/assets/container.css" type="text/css"/>
+      <link rel="stylesheet" href="${resURL}/scripts/yui/container/assets/skins/sam/container.css" type="text/css"/>
+      <link rel="stylesheet" href="${resURL}/scripts/yui/menu/assets/skins/sam/menu.css" type="text/css" />
+    </j:if>
 
     <l:hasPermission permission="${app.READ}">
       <link rel="search" type="application/opensearchdescription+xml" href="${rootURL}/opensearch.xml" title="Jenkins" />
@@ -173,7 +178,7 @@ THE SOFTWARE.
     <script src="${resURL}/jsbundles/sortable-drag-drop.js" type="text/javascript"/>
     <script src="${resURL}/jsbundles/app.js" type="text/javascript" defer="true" />
   </head>
-  <body id="jenkins" class="yui-skin-sam ${layoutType} jenkins-${h.version}" data-version="${h.version}" data-model-type="${it.class.name}">
+  <body id="jenkins" class="${removeYUI ? '' : 'yui-skin-sam'} ${layoutType} jenkins-${h.version}" data-version="${h.version}" data-model-type="${it.class.name}">
 
     <j:if test="${layoutType!='full-screen'}">
       <!-- for accessibility, skip the entire navigation bar and etc and go straight to the head of the content -->