Migrate haml button to our utility classes

ee/app/views/shared/milestones/_burndown.html.haml

@@ -25,5 +25,5 @@
           View your milestone's progress as a burndown chart.  Add both a start and a due date to
           this milestone and the chart will appear here, always up-to-date.
 
-        = link_to "Add start and due date", edit_milestone_path(milestone), class: 'btn'
+        = link_to "Add start and due date", edit_milestone_path(milestone), class: 'btn gl-button'
 = render 'shared/promotions/promote_burndown_charts', milestone: milestone