collapse the navbar on narrow windows

so it doesn't take up a bunch of vertical height when not in use

IPython/html/templates/notebook.html

@@ -59,6 +59,11 b' class="notebook_app"'
 <div id="menubar">
     <div id="menus" class="navbar navbar-default" role="navigation">
         <div class="container-fluid">
+          <button type="button" class="btn btn-default navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
+            <i class="fa fa-bars"></i>
+            <span class="navbar-text">Menu</span>
+          </button>
+          <div class="navbar-collapse collapse">
             <ul class="nav navbar-nav">
                 <li class="dropdown"><a href="#" class="dropdown-toggle" data-toggle="dropdown">File</a>
                     <ul id="file_menu" class="dropdown-menu">
@@ -270,6 +275,7 b' class="notebook_app"'
                     </ul>
                 </li>
             </ul>
+            </div>
             <ul class="nav navbar-nav navbar-right">
                 <li id="kernel_indicator">
                    <i id="kernel_indicator_icon"></i>