Simplify URL validation in related content

We were using `Setting["url"]` to verify the content belonged to the
application URL, but we can use `root_url` instead.

Note that means we need to include the port when filling in forms in the
tests, since in tests URL helpers like `polymorphic_url` don't include
the port, but a port is automatically added when actually making the
request.

app/controllers/related_contents_controller.rb

@@ -18,7 +18,7 @@ class RelatedContentsController < ApplicationController
     elsif related_content.duplicate?
       flash[:error] = t("related_content.error_duplicate")
     else
-      flash[:error] = t("related_content.error", url: Setting["url"])
+      flash[:error] = t("related_content.error", url: root_url)
     end
     redirect_to polymorphic_path(related_content.parent_relationable)
   end
@@ -41,7 +41,7 @@ class RelatedContentsController < ApplicationController
     end
 
     def valid_url?
-      params[:url].start_with?(Setting["url"])
+      params[:url].start_with?(root_url)
     end
 
     def child_relationable_params