Add star icon to star button

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

app/views/projects/buttons/_star.html.haml

 - if current_user
   = link_to toggle_star_namespace_project_path(@project.namespace, @project), class: 'btn star-btn toggle-star', method: :post, remote: true do
+    = icon('star')
     - if current_user.starred?(@project)
       Unstar
     - else