Use polymorphic_path instead of Relationable#url

The #url method in the models provides a feature that can be done using
native Rails features (polymorphic paths), is inconsistent (some models
have that method and some haven't), and only works for URLs in the
public area (but not in sections like administration or management).
Besides, models are already fat enough without introducing methods
related to controllers or routes.

app/controllers/related_contents_controller.rb

@@ -15,7 +15,7 @@ class RelatedContentsController < ApplicationController
     else
       flash[:error] = t("related_content.error", url: Setting["url"])
     end
-    redirect_to @relationable.url
+    redirect_to polymorphic_path(@relationable)
   end
 
   def score_positive