Hide top login/logout buttons on login/logout pages.

IPython/frontend/html/notebook/templates/layout.html

@@ -22,13 +22,19 b''
 
 <div id="header">
     <span id="ipython_notebook"><h1><img src='static/ipynblogo.png' alt='IPython Notebook'/></h1></span>
-    <span id="login_widget">
-      {% if logged_in %}
-        <button id="logout">Logout</button>
-      {% elif login_available and not logged_in %}
-        <button id="login">Login</button>
-      {% end %}
-    </span>
+
+    {% block login_widget %}
+
+      <span id="login_widget">
+        {% if logged_in %}
+          <button id="logout">Logout</button>
+        {% elif login_available and not logged_in %}
+          <button id="login">Login</button>
+        {% end %}
+      </span>
+
+    {% end %}
+
     {% block header %}
     {% end %}
 </div>

IPython/frontend/html/notebook/templates/login.html

@@ -13,6 +13,9 b''
 
 {% end %}
 
+{% block login_widget %}
+{% end %}
+
 {% block script %}
 <script type="text/javascript">
   $(document).ready(function() {

IPython/frontend/html/notebook/templates/logout.html

@@ -16,6 +16,9 b''
 
 {% end %}
 
+{% block login_widget %}
+{% end %}
+
 {% block script %}
 <script type="text/javascript">
   $(document).ready(function() {