Insert flex-box spacer in tree view header

Author: gnestor

notebook/static/base/less/page.less

@@ -58,13 +58,14 @@ body > #header {
 }
 
 #ipython_notebook {
-    flex: 1;
     padding-left: 0px;
     padding-top: (@navbar-height - @logo_height) / 2;
     padding-bottom: (@navbar-height - @logo_height) / 2;
 }
 
-
+.flex-spacer {
+    flex: 1;
+}
 
 #noscript {
      width: auto;

notebook/templates/tree.html

@@ -11,6 +11,9 @@
 data-server-root="{{server_root}}"
 {% endblock %}
 
+{% block headercontainer %}
+<span class="flex-spacer"></span>
+{% endblock %}
 
 {% block site %}