Incorporate @minrk feedback

Moving a few blocks to css classes that can be used through custom.css

notebook/templates/notebook.html

@@ -267,7 +267,7 @@ data-notebook-path="{{notebook_path}}"
 
                         {% for helplinks in sections %}
                             {% for link in helplinks %}
-                                <li><a {% block helplink_attributes %}{% endblock %} href="{{link[0]}}" {{'target="_blank" title="Opens in a new window"' if link[2]}}>
+                                <li><a class='help_link' href="{{link[0]}}" {{'target="_blank" title="Opens in a new window"' if link[2]}}>
                                 {{'<i class="fa fa-external-link menu-icon pull-right"></i>' if link[2]}}
                                 {{link[1]}}
                                 </a></li>

notebook/templates/tree.html

@@ -19,7 +19,7 @@ data-terminals-available="{{terminals_available}}"
       <ul id="tabs" class="nav nav-tabs">
         <li class="active"><a href="#notebooks" data-toggle="tab">Files</a></li>
         <li><a href="#running" data-toggle="tab">Running</a></li>
-        <li><a href="#clusters" data-toggle="tab" {% block clusters_tab_attributes %}{% endblock %} >Clusters</a></li>
+        <li><a href="#clusters" data-toggle="tab" class="clusters_tab_link" >Clusters</a></li>
       </ul>
       <div class="tab-content">
         <div id="notebooks" class="tab-pane active">