Attempt to fix broken submenus on smartphones

This is apparently a known issue in Bootstrap 2.3.x, with a few proposed
solutions. This is the simplest:
twbs/bootstrap#7968

webapp/views/layout.html

@@ -8,10 +8,13 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>{{=response.title or request.application}}</title>
     {{response.files.append(URL('static','css/bootstrap.css'))}}
+    {{response.files.append(URL('static','css/bootstrap-responsive.css'))}}
     <style type="text/css">
         /* TODO: add page-specific styles here */
+        .dropdown-backdrop{
+                position: static;
+        }
     </style>
-    {{response.files.append(URL('static','css/bootstrap-responsive.css'))}}
 
     <script type="text/javascript">
         /*