Merge pull request #3526 from jsoref/title

i18n/a11y: provide alt text for the jenkins logo and masthead

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

@@ -184,7 +184,7 @@ ${h.initPageVariables(context)}
       <div id="header">
         <div class="logo">
           <a id="jenkins-home-link" href="${rootURL}/">
-            <img id="jenkins-head-icon" src="${imagesURL}/headshot.png" alt="title" />
+            <img id="jenkins-head-icon" src="${imagesURL}/headshot.png" alt="[${%jenkinshead.alt}]" />
             <img id="jenkins-name-icon" src="${imagesURL}/title.png" alt="${%title}" width="139" height="34" />
           </a>
         </div>

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

@@ -23,3 +23,5 @@
 # do not localize unless a localized wiki page really exists.
 searchBox.url=https://jenkins.io/redirect/search-box
 logout=log out
+title=Jenkins
+jenkinshead.alt=Jenkins