Style project milestones page

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

app/views/projects/milestones/index.html.haml

 - page_title "Milestones"
-.pull-right
+= render 'shared/milestones_filter'
+
+.gray-content-block
+  .pull-right
     - if can? current_user, :admin_milestone, @project
       = link_to new_namespace_project_milestone_path(@project.namespace, @project), class: "pull-right btn btn-new", title: "New Milestone" do
         %i.fa.fa-plus
         New Milestone
-= render 'shared/milestones_filter'
+  .oneline
+    Milestone allows you to group issues and set due date for it
 
 .milestones
-  .panel.panel-default
-    %ul.well-list
+  %ul.content-list
     = render @milestones
 
     - if @milestones.blank?