Add "No Milestone" option to issue milestone select

app/helpers/issues_helper.rb

@@ -50,8 +50,10 @@ module IssuesHelper
   end
 
   def milestone_options(object)
-    options_from_collection_for_select(object.project.milestones.active,
-                                       'id', 'title', object.milestone_id)
+    milestones = object.project.milestones.active.to_a
+    milestones.unshift(Milestone::None)
+
+    options_from_collection_for_select(milestones, 'id', 'title', object.milestone_id)
   end
 
   def issue_box_class(item)