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.

spec/support/common_actions.rb

@@ -19,7 +19,11 @@ module CommonActions
   include Verifications
 
   def app_host
-    "#{Capybara.app_host}:#{Capybara::Server.ports.values.last}"
+    "#{Capybara.app_host}:#{app_port}"
+  end
+
+  def app_port
+    Capybara::Server.ports.values.last
   end
 
   def fill_in_signup_form(email = "manuela@consul.dev", password = "judgementday")