updates buttons in app/views/groups/milestones directory

app/views/groups/milestones/_form.html.haml

@@ -19,8 +19,8 @@
 
   .form-actions
     - if @milestone.new_record?
-      = f.submit 'Create milestone', class: "btn-success btn", data: { qa_selector: "create_milestone_button" }
-      = link_to "Cancel", group_milestones_path(@group), class: "btn btn-cancel"
+      = f.submit 'Create milestone', class: "btn-success gl-button btn", data: { qa_selector: "create_milestone_button" }
+      = link_to "Cancel", group_milestones_path(@group), class: "btn gl-button btn-cancel"
     - else
-      = f.submit 'Update milestone', class: "btn-success btn"
-      = link_to "Cancel", group_milestone_path(@group, @milestone), class: "btn btn-cancel"
+      = f.submit 'Update milestone', class: "btn-success gl-button btn"
+      = link_to "Cancel", group_milestone_path(@group, @milestone), class: "btn gl-button btn-cancel"

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

@@ -8,7 +8,7 @@
     = render 'shared/milestones/search_form'
     = render 'shared/milestones_sort_dropdown'
     - if can?(current_user, :admin_milestone, @group)
-      = link_to "New milestone", new_group_milestone_path(@group), class: "btn btn-success", data: { qa_selector: "new_group_milestone_link" }
+      = link_to "New milestone", new_group_milestone_path(@group), class: "btn gl-button btn-success", data: { qa_selector: "new_group_milestone_link" }
 
 .milestones
   %ul.content-list