Use sign icon style for help link

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/layouts/_head_panel.html.haml

@@ -22,7 +22,7 @@
           %li
             = link_to help_path, title: 'Help', class: 'has_bottom_tooltip',
                'data-original-title' => 'Help'  do
-              %i.icon-question
+              %i.icon-question-sign
           %li
             = link_to public_root_path, title: "Public area", class: 'has_bottom_tooltip', 'data-original-title' => 'Public area' do
               %i.icon-globe