Add milestone observer

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

app/observers/milestone_observer.rb

+class MilestoneObserver < BaseObserver
+  def after_create(milestone)
+    event_service.open_milestone(milestone, current_user)
+  end
+
+  def after_close(milestone, transition)
+    event_service.close_milestone(milestone, current_user)
+  end
+
+  def after_reopen(milestone, transition)
+    event_service.reopen_milestone(milestone, current_user)
+  end
+end