Fix admin page with handlebars 4

src/static/templates/admin/base.hbs

@@ -117,7 +117,7 @@
         </div>
     </nav>
 
-    {{> (page_content) }}
+    {{> (lookup this "page_content") }}
 
     <!-- This script needs to be at the bottom, else it will fail! -->
     <script>