Add branding to login screen

This demonstrates branding of the login/register screen made possible via jenkinsci/jenkins#8462

pom.xml

@@ -44,7 +44,7 @@ THE SOFTWARE.
     <properties>
         <changelist>999999-SNAPSHOT</changelist>
         <gitHubRepo>jenkinsci/design-library-plugin</gitHubRepo>
-        <jenkins.version>2.415</jenkins.version>
+        <jenkins.version>2.422-rc34164.1202c5b_f2391</jenkins.version>
         <node.version>18.17.1</node.version>
         <npm.version>9.8.1</npm.version>
     </properties>

src/main/java/io/jenkins/plugins/designlibrary/DesignLibrarySimplePageDecorator.java

@@ -0,0 +1,8 @@
+package io.jenkins.plugins.designlibrary;
+
+import hudson.Extension;
+import jenkins.model.SimplePageDecorator;
+
+@Extension
+public class DesignLibrarySimplePageDecorator extends SimplePageDecorator {
+}

src/main/resources/io/jenkins/plugins/designlibrary/DesignLibrarySimplePageDecorator/simple-branding.jelly

@@ -0,0 +1,31 @@
+<!--
+The MIT License
+
+Copyright (c) 2023, CloudBees, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
+-->
+<?jelly escape-by-default='true'?>
+<j:jelly xmlns:j="jelly:core">
+  <link rel="stylesheet" href="${resURL}/plugin/design-library/css/branding.css" type="text/css" />
+  <section class="app-sign-in-register__branding">
+    <div class="jdl-branding"></div>
+    <img src="${resURL}/plugin/design-library/images/design-library.svg" alt="${%logo}"/>
+  </section>
+</j:jelly>

src/main/webapp/css/branding.css

@@ -0,0 +1,14 @@
+.jdl-branding {
+  position: absolute;
+  inset: 0;
+  z-index: 1;
+  background: linear-gradient(
+    99.6deg,
+    rgb(112, 128, 152) 10.6%,
+    rgb(242, 227, 234) 32.9%,
+    rgb(234, 202, 213) 52.7%,
+    rgb(220, 227, 239) 72.8%,
+    rgb(185, 205, 227) 81.1%,
+    rgb(154, 180, 212) 102.4%
+  );
+}