Make the actual labels on Labels#index links

Now the user can click the labels themselves or the "X open issues"
text.

app/views/projects/labels/_label.html.haml

 %li{id: dom_id(label)}
-  = render_colored_label(label)
+  = link_to_label(label)
   .pull-right
     %strong.append-right-20
       = link_to_label(label) do