Backport PR #8215: add header_buttons block to page template

allows easier access to adding buttons next to the logout button in custom templates.

IPython/html/templates/page.html

@@ -85,6 +85,8 b''
   <div id="header-container" class="container">
   <div id="ipython_notebook" class="nav navbar-brand pull-left"><a href="{{default_url}}" title='dashboard'>{% block logo %}<img src='{{static_url("base/images/logo.png") }}' alt='Jupyter Notebook'/>{% endblock %}</a></div>
 
+  {% block header_buttons %}
+
   {% block login_widget %}
 
     <span id="login_widget">
@@ -97,6 +99,8 b''
 
   {% endblock %}
 
+  {% endblock header_buttons %}
+
   {% block headercontainer %}
   {% endblock %}
   </div>