Add plug-in's public folder to core's

lib/open_project/costs/engine.rb

@@ -175,6 +175,12 @@ module OpenProject::Costs
       ActionView::Helpers::NumberHelper.send(:include, OpenProject::Costs::Patches::NumberHelperPatch)
     end
 
+    # Initializer to combine this engines static assets with the static assets of the hosting site.
+    #  Thanks to http://jbavari.github.io/blog/2013/10/26/rev-up-your-rails-engine-for-static-assets/
+    initializer "static assets" do |app|
+      app.middleware.insert_before(::ActionDispatch::Static, ::ActionDispatch::Static, "#{root}/public")
+    end
+
     config.to_prepare do
       # loading the class so that acts_as_journalized gets registered
       VariableCostObject