Introduce headerButtonGroup content_block

Change-Id: I5a3ca7d46befc702c0bda2203498e76cdebe17f4
diff --git a/templates/partial/header.html.ep b/templates/partial/header.html.ep
index 349104e..5f1433d 100644
--- a/templates/partial/header.html.ep
+++ b/templates/partial/header.html.ep
@@ -1,14 +1,7 @@
 <header>
   <%= link_to 'index', class => 'logo', begin %><h1><span><%= title() // loc('korap_desc') %></span></h1><% end %>
   <div class="button top">
-
-% if (stash('user')) {
-   %# TODO: CSRF protection
-   <a href="<%= url_for 'logout' %>"
-      class="logout"
-      title="<%= loc 'logout' %>: <%= stash('user') %>"><span><%= loc 'logout' %></span></a>
-% };
-
+    %= content_block 'headerButtonGroup'
   </div>
   <form autocomplete="off" action="<%= url_for 'index' %>" id="searchform">
     <div id="searchbar">