[JENKINS-62433] Read-only system message (#4745)

* Read-only system message

* More robust js
Co-authored-by: Nres0nance <res0nance@users.noreply.github.com>

core/src/main/resources/jenkins/model/Jenkins/configure.jelly

@@ -40,7 +40,8 @@ THE SOFTWARE.
         ${it.rootDir}
       </f:entry>
       <f:entry title="${%System Message}" help="/help/system-config/systemMessage.html">
-        <f:textarea name="system_message" value="${it.systemMessage}"
+        <j:set var="readOnlyMode" value="${!h.hasPermission(app.MANAGE)}" />
+        <f:textarea name="system_message" value="${it.systemMessage}" disabled="${readOnlyMode?'true':null}" readonly="${readOnlyMode?'true':null}"
                     codemirror-mode="${app.markupFormatter.codeMirrorMode}" codemirror-config="${app.markupFormatter.codeMirrorConfig}" previewEndpoint="/markupFormatter/previewDescription"/>
       </f:entry>
 

core/src/main/resources/lib/form/textarea/textarea.js

@@ -48,7 +48,14 @@ Behaviour.specify("DIV.textarea-preview-container", 'textarea', 100, function (e
         showPreview.onclick = function() {
             // Several TEXTAREAs may exist if CodeMirror is enabled. The first one has reference to the CodeMirror object.
             var textarea = e.parentNode.getElementsByTagName("TEXTAREA")[0];
-            var text = textarea.codemirrorObject ? textarea.codemirrorObject.getValue() : textarea.value;
+            var text = "";
+            //Textarea object will be null if the text area is disabled.
+            if (textarea == null) {
+                textarea = e.parentNode.getElementsByClassName("jenkins-readonly")[0];
+                text = textarea != null ? textarea.innerText : "";
+            } else {
+                text = textarea.codemirrorObject ? textarea.codemirrorObject.getValue() : textarea.value;
+            }
             var render = function(txt) {
                 $(hidePreview).show();
                 $(previewDiv).show();